diff --git a/includes/class-mysql.php b/includes/class-mysql.php
index d2ca295..c25c183 100644
--- a/includes/class-mysql.php
+++ b/includes/class-mysql.php
@@ -4,6 +4,7 @@
  * Connect to DB
  *
  * @since 1.0
+ * @return \YOURLS\Database\YDB
  */
 function yourls_db_connect() {
     global $ydb;
@@ -101,6 +102,7 @@ function yourls_get_db() {
  *
  * @since 1.7.10
  * @param  mixed $db    Either a \YOURLS\Database\YDB instance, or anything. If null, the function will unset $ydb
+ * @return void
  */
 function yourls_set_db($db) {
     global $ydb;
diff --git a/includes/functions-api.php b/includes/functions-api.php
index baf3cb1..45395ac 100644
--- a/includes/functions-api.php
+++ b/includes/functions-api.php
@@ -163,8 +163,12 @@ function yourls_api_output( $mode, $output, $send_headers = true, $echo = true )
 /**
  * Return array for API stat requests
  *
+ * @param string $filter  either "top", "bottom" , "rand" or "last"
+ * @param int    $limit   maximum number of links to return
+ * @param int    $start   offset
+ * @return array
  */
-function yourls_api_stats( $filter = 'top', $limit = 10, $start = 0 ) {
+function yourls_api_stats($filter = 'top', $limit = 10, $start = 0 ) {
 	$return = yourls_get_stats( $filter, $limit, $start );
 	$return['simple']  = 'Need either XML or JSON format for stats';
 	$return['message'] = 'success';
@@ -174,6 +178,7 @@ function yourls_api_stats( $filter = 'top', $limit = 10, $start = 0 ) {
 /**
  * Return array for counts of shorturls and clicks
  *
+ * @return array
  */
 function yourls_api_db_stats() {
 	$return = array(
@@ -189,6 +194,8 @@ function yourls_api_db_stats() {
 /**
  * Return array for API stat requests
  *
+ * @param string $shorturl  Short URL to check
+ * @return array
  */
 function yourls_api_url_stats( $shorturl ) {
 	$keyword = str_replace( yourls_get_yourls_site() . '/' , '', $shorturl ); // accept either 'http://ozh.in/abc' or 'abc'
@@ -202,6 +209,8 @@ function yourls_api_url_stats( $shorturl ) {
 /**
  * Expand short url to long url
  *
+ * @param string $shorturl  Short URL to expand
+ * @return array
  */
 function yourls_api_expand( $shorturl ) {
 	$keyword = str_replace( yourls_get_yourls_site() . '/' , '', $shorturl ); // accept either 'http://ozh.in/abc' or 'abc'
diff --git a/includes/functions-auth.php b/includes/functions-auth.php
index 52a677a..2fd76d9 100644
--- a/includes/functions-auth.php
+++ b/includes/functions-auth.php
@@ -7,6 +7,7 @@
 /**
  * Show login form if required
  *
+ * @return void
  */
 function yourls_maybe_require_auth() {
 	if( yourls_is_private() ) {
@@ -37,7 +38,7 @@ function yourls_is_valid_user() {
         // The logout nonce is associated to fake user 'logout' since at this point we don't know the real user
         yourls_verify_nonce('admin_logout', $_REQUEST['nonce'], 'logout');
 		yourls_do_action( 'logout' );
-		yourls_store_cookie( null );
+		yourls_store_cookie( '' );
 		return yourls__( 'Logged out successfully' );
 	}
 
@@ -124,6 +125,7 @@ function yourls_is_valid_user() {
 /**
  * Check auth against list of login=>pwd. Sets user if applicable, returns bool
  *
+ * @return bool  true if login/pwd pair is valid (and sets user if applicable), false otherwise
  */
 function yourls_check_username_password() {
 	global $yourls_user_passwords;
@@ -143,8 +145,11 @@ function yourls_check_username_password() {
 /**
  * Check a submitted password sent in plain text against stored password which can be a salted hash
  *
+ * @param string $user
+ * @param string $submitted_password
+ * @return bool
  */
-function yourls_check_password_hash( $user, $submitted_password ) {
+function yourls_check_password_hash($user, $submitted_password ) {
 	global $yourls_user_passwords;
 
 	if( !isset( $yourls_user_passwords[ $user ] ) )
@@ -173,11 +178,15 @@ function yourls_check_password_hash( $user, $submitted_password ) {
  * @return true|string  if overwrite was successful, an error message otherwise
  */
 function yourls_hash_passwords_now( $config_file ) {
-	if( !is_readable( $config_file ) )
-		return 'cannot read file'; // not sure that can actually happen...
+	if( !is_readable( $config_file ) ) {
+        yourls_debug_log( 'Cannot hash passwords: cannot read file ' . $config_file );
+        return 'cannot read file'; // not sure that can actually happen...
+    }
 
-	if( !is_writable( $config_file ) )
+	if( !is_writable( $config_file ) ) {
+        yourls_debug_log( 'Cannot hash passwords: cannot write file ' . $config_file );
 		return 'cannot write file';
+    }
 
     $yourls_user_passwords = [];
 	// Include file to read value of $yourls_user_passwords
@@ -188,11 +197,16 @@ function yourls_hash_passwords_now( $config_file ) {
 	error_reporting( $errlevel );
 
 	$configdata = file_get_contents( $config_file );
-	if( $configdata == false )
-		return 'could not read file';
+
+    if( $configdata == false ) {
+        yourls_debug_log('Cannot hash passwords: file_get_contents() false with ' . $config_file);
+        return 'could not read file';
+    }
 
 	$to_hash = 0; // keep track of number of passwords that need hashing
 	foreach ( $yourls_user_passwords as $user => $password ) {
+        // avoid "deprecated" warning when password is null -- see test case in tests/data/auth/preg_replace_problem.php
+        $password ??= '';
 		if ( !yourls_has_phpass_password( $user ) && !yourls_has_md5_password( $user ) ) {
 			$to_hash++;
 			$hash = yourls_phpass_hash( $password );
@@ -211,8 +225,10 @@ function yourls_hash_passwords_now( $config_file ) {
 		}
 	}
 
-	if( $to_hash == 0 )
-		return 0; // There was no password to encrypt
+	if( $to_hash == 0 ) {
+        yourls_debug_log('Cannot hash passwords: no password found in ' . $config_file);
+        return 'no password found';
+    }
 
 	$success = file_put_contents( $config_file, $configdata );
 	if ( $success === FALSE ) {
@@ -320,6 +336,7 @@ function yourls_has_phpass_password( $user ) {
 /**
  * Check auth against encrypted COOKIE data. Sets user if applicable, returns bool
  *
+ * @return bool true if authenticated, false otherwise
  */
 function yourls_check_auth_cookie() {
 	global $yourls_user_passwords;
@@ -406,6 +423,8 @@ function yourls_check_signature() {
 /**
  * Generate secret signature hash
  *
+ * @param false|string $username  Username to generate signature for, or false to use current user
+ * @return string                 Signature
  */
 function yourls_auth_signature( $username = false ) {
 	if( !$username && defined('YOURLS_USER') ) {
@@ -417,6 +436,8 @@ function yourls_auth_signature( $username = false ) {
 /**
  * Check if timestamp is not too old
  *
+ * @param int $time  Timestamp to check
+ * @return bool      True if timestamp is valid
  */
 function yourls_check_timestamp( $time ) {
 	$now = time();
@@ -427,9 +448,10 @@ function yourls_check_timestamp( $time ) {
 /**
  * Store new cookie. No $user will delete the cookie.
  *
- * @param mixed $user  String, user login, or null to delete cookie
+ * @param string $user  User login, or empty string to delete cookie
+ * @return void
  */
-function yourls_store_cookie( $user = null ) {
+function yourls_store_cookie( $user = '' ) {
 
     // No user will delete the cookie with a cookie time from the past
 	if( !$user ) {
@@ -463,7 +485,6 @@ function yourls_store_cookie( $user = null ) {
  *
  * @see https://github.com/GoogleChromeLabs/samesite-examples/blob/master/php.md
  * @see https://stackoverflow.com/a/59654832/36850
- * @see https://3v4l.org/uKEtH for compat tests
  * @see https://www.php.net/manual/en/function.setcookie.php
  *
  * @since  1.7.7
@@ -479,24 +500,21 @@ function yourls_store_cookie( $user = null ) {
 function yourls_setcookie($name, $value, $expire, $path, $domain, $secure, $httponly) {
     $samesite = yourls_apply_filter('setcookie_samesite', 'Lax' );
 
-    if (PHP_VERSION_ID < 70300) {
-        return(setcookie($name, $value, $expire, "$path; samesite=$samesite", $domain, $secure, $httponly));
-    }
-    else {
-        return(setcookie($name, $value, array(
-            'expires'  => $expire,
-            'path'     => $path,
-            'domain'   => $domain,
-            'samesite' => $samesite,
-            'secure'   => $secure,
-            'httponly' => $httponly,
-        )));
-    }
+    return(setcookie($name, $value, array(
+        'expires'  => $expire,
+        'path'     => $path,
+        'domain'   => $domain,
+        'samesite' => $samesite,
+        'secure'   => $secure,
+        'httponly' => $httponly,
+    )));
 }
 
 /**
  * Set user name
  *
+ * @param string $user  Username
+ * @return void
  */
 function yourls_set_user( $user ) {
 	if( !defined( 'YOURLS_USER' ) )
@@ -554,7 +572,7 @@ function yourls_cookie_name() {
  * @return string          cookie value
  */
 function yourls_cookie_value( $user ) {
-	return yourls_apply_filter( 'set_cookie_value', yourls_salt( $user ), $user );
+	return yourls_apply_filter( 'set_cookie_value', yourls_salt( $user ?? '' ), $user );
 }
 
 /**
@@ -562,6 +580,7 @@ function yourls_cookie_value( $user ) {
  *
  * Actually, this returns a float: ceil rounds up a value but is of type float, see https://www.php.net/ceil
  *
+ * @return float
  */
 function yourls_tick() {
 	return ceil( time() / yourls_get_nonce_life() );
@@ -598,8 +617,11 @@ function yourls_hmac_algo() {
 /**
  * Create a time limited, action limited and user limited token
  *
+ * @param string $action      Action to create nonce for
+ * @param false|string $user  Optional user string, false for current user
+ * @return string             Nonce token
  */
-function yourls_create_nonce( $action, $user = false ) {
+function yourls_create_nonce($action, $user = false ) {
 	if( false === $user ) {
         $user = defined('YOURLS_USER') ? YOURLS_USER : '-1';
     }
@@ -610,10 +632,15 @@ function yourls_create_nonce( $action, $user = false ) {
 }
 
 /**
- * Create a nonce field for inclusion into a form
+ * Echoes or returns a nonce field for inclusion into a form
  *
+ * @param string $action      Action to create nonce for
+ * @param string $name        Optional name of nonce field -- defaults to 'nonce'
+ * @param false|string $user  Optional user string, false if unspecified
+ * @param bool $echo          True to echo, false to return nonce field
+ * @return string             Nonce field
  */
-function yourls_nonce_field( $action, $name = 'nonce', $user = false, $echo = true ) {
+function yourls_nonce_field($action, $name = 'nonce', $user = false, $echo = true ) {
 	$field = '<input type="hidden" id="'.$name.'" name="'.$name.'" value="'.yourls_create_nonce( $action, $user ).'" />';
 	if( $echo )
 		echo $field."\n";
@@ -623,8 +650,13 @@ function yourls_nonce_field( $action, $name = 'nonce', $user = false, $echo = tr
 /**
  * Add a nonce to a URL. If URL omitted, adds nonce to current URL
  *
+ * @param string $action      Action to create nonce for
+ * @param string $url         Optional URL to add nonce to -- defaults to current URL
+ * @param string $name        Optional name of nonce field -- defaults to 'nonce'
+ * @param false|string $user  Optional user string, false if unspecified
+ * @return string             URL with nonce added
  */
-function yourls_nonce_url( $action, $url = false, $name = 'nonce', $user = false ) {
+function yourls_nonce_url($action, $url = false, $name = 'nonce', $user = false ) {
 	$nonce = yourls_create_nonce( $action, $user );
 	return yourls_add_query_arg( $name, $nonce, $url );
 }
@@ -632,11 +664,16 @@ function yourls_nonce_url( $action, $url = false, $name = 'nonce', $user = false
 /**
  * Check validity of a nonce (ie time span, user and action match).
  *
- * Returns true if valid, dies otherwise (yourls_die() or die($return) if defined)
- * if $nonce is false or unspecified, it will use $_REQUEST['nonce']
+ * Returns true if valid, dies otherwise (yourls_die() or die($return) if defined).
+ * If $nonce is false or unspecified, it will use $_REQUEST['nonce']
  *
+ * @param string $action
+ * @param false|string $nonce  Optional, string: nonce value, or false to use $_REQUEST['nonce']
+ * @param false|string $user   Optional, string user, false for current user
+ * @param string $return       Optional, string: message to die with if nonce is invalid
+ * @return bool|void           True if valid, dies otherwise
  */
-function yourls_verify_nonce( $action, $nonce = false, $user = false, $return = '' ) {
+function yourls_verify_nonce($action, $nonce = false, $user = false, $return = '' ) {
 	// Get user
 	if( false === $user ) {
         $user = defined('YOURLS_USER') ? YOURLS_USER : '-1';
@@ -668,7 +705,7 @@ function yourls_verify_nonce( $action, $nonce = false, $user = false, $return =
  * Check if YOURLS_USER comes from environment variables
  *
  * @since 1.8.2
- * @return bool     true if YOURLS_USER and YOURLS_PASSWORD are defined as environment variables
+ * @return bool  true if YOURLS_USER and YOURLS_PASSWORD are defined as environment variables
  */
 function yourls_is_user_from_env() {
 	return yourls_apply_filter('is_user_from_env', getenv('YOURLS_USER') && getenv('YOURLS_PASSWORD'));
diff --git a/includes/functions-debug.php b/includes/functions-debug.php
index 85b4f0b..1aeee65 100644
--- a/includes/functions-debug.php
+++ b/includes/functions-debug.php
@@ -34,6 +34,7 @@ function yourls_get_debug_log() {
 /**
  * Get number of SQL queries performed
  *
+ * @return int
  */
 function yourls_get_num_queries() {
 	return yourls_apply_filter( 'get_num_queries', yourls_get_db()->get_num_queries() );
@@ -44,6 +45,7 @@ function yourls_get_num_queries() {
  *
  * @since 1.7.3
  * @param bool $bool Debug on or off
+ * @return void
  */
 function yourls_debug_mode( $bool ) {
     // log queries if true
diff --git a/includes/functions-deprecated.php b/includes/functions-deprecated.php
index a481f77..bbc4f3c 100644
--- a/includes/functions-deprecated.php
+++ b/includes/functions-deprecated.php
@@ -5,10 +5,61 @@
  *
  * Note to devs: when deprecating a function, move it here. Then check all the places
  * in core that might be using it, including core plugins.
+ *
+ * Usage :  yourls_deprecated_function( 'function_name', 'version', 'replacement' );
+ * Output:  "{function_name} is deprecated since version {version}! Use {replacement} instead."
+ *
+ * Usage :  yourls_deprecated_function( 'function_name', 'version' );
+ * Output:  "{function_name} is deprecated since version {version} with no alternative available."
+ *
+ * @see yourls_deprecated_function()
  */
 
 // @codeCoverageIgnoreStart
 
+/**
+ * Return current admin page, or null if not an admin page. Was not used anywhere.
+ *
+ * @return mixed string if admin page, null if not an admin page
+ * @since 1.6
+ * @deprecated 1.9.1
+ */
+function yourls_current_admin_page() {
+    yourls_deprecated_function( __FUNCTION__, '1.9.1' );
+    if( yourls_is_admin() ) {
+        $current = substr( yourls_get_request(), 6 );
+        if( $current === false )
+            $current = 'index.php'; // if current page is http://sho.rt/admin/ instead of http://sho.rt/admin/index.php
+
+        return $current;
+    }
+    return null;
+}
+
+/**
+ * PHP emulation of JS's encodeURI
+ *
+ * @link https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/encodeURI
+ * @deprecated 1.9.1
+ * @param string $url
+ * @return string
+ */
+function yourls_encodeURI($url) {
+    yourls_deprecated_function( __FUNCTION__, '1.9.1', '' );
+    // Decode URL all the way
+    $result = yourls_rawurldecode_while_encoded( $url );
+    // Encode once
+    $result = strtr( rawurlencode( $result ), array (
+        '%3B' => ';', '%2C' => ',', '%2F' => '/', '%3F' => '?', '%3A' => ':', '%40' => '@',
+        '%26' => '&', '%3D' => '=', '%2B' => '+', '%24' => '$', '%21' => '!', '%2A' => '*',
+        '%27' => '\'', '%28' => '(', '%29' => ')', '%23' => '#',
+    ) );
+    // @TODO:
+    // Known limit: this will most likely break IDN URLs such as http://www.académie-française.fr/
+    // To fully support IDN URLs, advocate use of a plugin.
+    return yourls_apply_filter( 'encodeURI', $result, $url );
+}
+
 /**
  * Check if a file is a plugin file
  *
diff --git a/includes/functions-formatting.php b/includes/functions-formatting.php
index 08ac8d9..83fc396 100644
--- a/includes/functions-formatting.php
+++ b/includes/functions-formatting.php
@@ -7,15 +7,18 @@
 /**
  * Convert an integer (1337) to a string (3jk).
  *
+ * @param int $num       Number to convert
+ * @param string $chars  Characters to use for conversion
+ * @return string        Converted number
  */
-function yourls_int2string( $num, $chars = null ) {
+function yourls_int2string($num, $chars = null) {
 	if( $chars == null )
 		$chars = yourls_get_shorturl_charset();
 	$string = '';
 	$len = strlen( $chars );
 	while( $num >= $len ) {
-		$mod = bcmod( $num, $len );
-		$num = bcdiv( $num, $len );
+		$mod = bcmod( (string)$num, (string)$len );
+		$num = bcdiv( (string)$num, (string)$len );
 		$string = $chars[ $mod ] . $string;
 	}
 	$string = $chars[ intval( $num ) ] . $string;
@@ -26,8 +29,11 @@ function yourls_int2string( $num, $chars = null ) {
 /**
  * Convert a string (3jk) to an integer (1337)
  *
+ * @param string $string  String to convert
+ * @param string $chars   Characters to use for conversion
+ * @return string         Number (as a string)
  */
-function yourls_string2int( $string, $chars = null ) {
+function yourls_string2int($string, $chars = null) {
 	if( $chars == null )
 		$chars = yourls_get_shorturl_charset();
 	$integer = 0;
@@ -36,7 +42,7 @@ function yourls_string2int( $string, $chars = null ) {
 	$inputlen = strlen( $string );
 	for ($i = 0; $i < $inputlen; $i++) {
 		$index = strpos( $chars, $string[$i] );
-		$integer = bcadd( $integer, bcmul( $index, bcpow( $baselen, $i ) ) );
+		$integer = bcadd( (string)$integer, bcmul( (string)$index, bcpow( (string)$baselen, (string)$i ) ) );
 	}
 
 	return yourls_apply_filter( 'string2int', $integer, $string, $chars );
@@ -48,7 +54,6 @@ function yourls_string2int( $string, $chars = null ) {
  * @since   1.8.3
  * @param  string $prefix   Optional prefix
  * @return string           The unique string
- *
  */
 function yourls_unique_element_id($prefix = 'yid') {
     static $id_counter = 0;
@@ -139,8 +144,11 @@ function yourls_sanitize_url_safe( $unsafe_url, $protocols = array() ) {
  *
  * Stolen from WP's _deep_replace
  *
+ * @param string|array $search   Needle, or array of needles.
+ * @param string       $subject  Haystack.
+ * @return string                The string with the replaced values.
  */
-function yourls_deep_replace( $search, $subject ){
+function yourls_deep_replace($search, $subject ){
 	$found = true;
 	while($found) {
 		$found = false;
@@ -158,24 +166,31 @@ function yourls_deep_replace( $search, $subject ){
 /**
  * Make sure an integer is a valid integer (PHP's intval() limits to too small numbers)
  *
+ * @param int $int  Integer to check
+ * @return string   Integer as a string
  */
-function yourls_sanitize_int( $int ) {
+function yourls_sanitize_int($int ) {
 	return ( substr( preg_replace( '/[^0-9]/', '', strval( $int ) ), 0, 20 ) );
 }
 
 /**
  * Sanitize an IP address
+ * No check on validity, just return a sanitized string
  *
+ * @param string $ip  IP address
+ * @return string     IP address
  */
-function yourls_sanitize_ip( $ip ) {
+function yourls_sanitize_ip($ip ) {
 	return preg_replace( '/[^0-9a-fA-F:., ]/', '', $ip );
 }
 
 /**
  * Make sure a date is m(m)/d(d)/yyyy, return false otherwise
  *
+ * @param string $date  Date to check
+ * @return false|mixed  Date in format m(m)/d(d)/yyyy or false if invalid
  */
-function yourls_sanitize_date( $date ) {
+function yourls_sanitize_date($date ) {
 	if( !preg_match( '!^\d{1,2}/\d{1,2}/\d{4}$!' , $date ) ) {
 		return false;
 	}
@@ -185,18 +200,24 @@ function yourls_sanitize_date( $date ) {
 /**
  * Sanitize a date for SQL search. Return false if malformed input.
  *
+ * @param string $date   Date
+ * @return false|string  String in Y-m-d format for SQL search or false if malformed input
  */
-function yourls_sanitize_date_for_sql( $date ) {
+function yourls_sanitize_date_for_sql($date) {
 	if( !yourls_sanitize_date( $date ) )
 		return false;
 	return date( 'Y-m-d', strtotime( $date ) );
 }
 
 /**
- * Return trimmed string
+ * Return trimmed string, optionally append '[...]' if string is too long
  *
+ * @param string $string  String to trim
+ * @param int $length     Maximum length of string
+ * @param string $append  String to append if trimmed
+ * @return string         Trimmed string
  */
-function yourls_trim_long_string( $string, $length = 60, $append = '[...]' ) {
+function yourls_trim_long_string($string, $length = 60, $append = '[...]') {
 	$newstring = $string;
     if ( mb_strlen( $newstring ) > $length ) {
         $newstring = mb_substr( $newstring, 0, $length - mb_strlen( $append ), 'UTF-8' ) . $append;
@@ -225,8 +246,10 @@ function yourls_sanitize_version( $version ) {
 /**
  * Sanitize a filename (no Win32 stuff)
  *
+ * @param string $file  File name
+ * @return string|null  Sanitized file name (or null if it's just backslashes, ok...)
  */
-function yourls_sanitize_filename( $file ) {
+function yourls_sanitize_filename($file) {
 	$file = str_replace( '\\', '/', $file ); // sanitize for Win32 installs
 	$file = preg_replace( '|/+|' ,'/', $file ); // remove any duplicate slash
 	return $file;
@@ -235,8 +258,10 @@ function yourls_sanitize_filename( $file ) {
 /**
  * Check if a string seems to be UTF-8. Stolen from WP.
  *
+ * @param string $str  String to check
+ * @return bool        Whether string seems valid UTF-8
  */
-function yourls_seems_utf8( $str ) {
+function yourls_seems_utf8($str) {
 	$length = strlen( $str );
 	for ( $i=0; $i < $length; $i++ ) {
 		$c = ord( $str[ $i ] );
@@ -417,6 +442,8 @@ function yourls_specialchars_decode( $string, $quote_style = ENT_NOQUOTES ) {
 	$others = array( '&lt;'   => '<', '&#060;'  => '<', '&gt;'   => '>', '&#062;'  => '>', '&amp;'  => '&', '&#038;'  => '&', '&#x26;' => '&' );
 	$others_preg = array( '/&#0*60;/'  => '&#060;', '/&#0*62;/'  => '&#062;', '/&#0*38;/'  => '&#038;', '/&#x0*26;/i' => '&#x26;' );
 
+    $translation = $translation_preg = [];
+
 	if ( $quote_style === ENT_QUOTES ) {
 		$translation = array_merge( $single, $double, $others );
 		$translation_preg = array_merge( $single_preg, $double_preg, $others_preg );
@@ -654,40 +681,16 @@ function yourls_esc_textarea( $text ) {
 	return yourls_apply_filter( 'esc_textarea', $safe_text, $text );
 }
 
-
-/**
-* PHP emulation of JS's encodeURI
-*
-* @link https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/encodeURI
-* @param $url
-* @return string
-*/
-function yourls_encodeURI( $url ) {
-	// Decode URL all the way
-	$result = yourls_rawurldecode_while_encoded( $url );
-	// Encode once
-	$result = strtr( rawurlencode( $result ), array (
-        '%3B' => ';', '%2C' => ',', '%2F' => '/', '%3F' => '?', '%3A' => ':', '%40' => '@',
-		'%26' => '&', '%3D' => '=', '%2B' => '+', '%24' => '$', '%21' => '!', '%2A' => '*',
-		'%27' => '\'', '%28' => '(', '%29' => ')', '%23' => '#',
-    ) );
-	// @TODO:
-	// Known limit: this will most likely break IDN URLs such as http://www.académie-française.fr/
-	// To fully support IDN URLs, advocate use of a plugin.
-	return yourls_apply_filter( 'encodeURI', $result, $url );
-}
-
 /**
  * Adds backslashes before letters and before a number at the start of a string. Stolen from WP.
  *
  * @since 1.6
- *
  * @param string $string Value to which backslashes will be added.
  * @return string String with backslashes inserted.
  */
 function yourls_backslashit($string) {
-    $string = preg_replace('/^([0-9])/', '\\\\\\\\\1', $string);
-    $string = preg_replace('/([a-z])/i', '\\\\\1', $string);
+    $string = preg_replace('/^([0-9])/', '\\\\\\\\\1', (string)$string);
+    $string = preg_replace('/([a-z])/i', '\\\\\1', (string)$string);
     return $string;
 }
 
@@ -745,7 +748,7 @@ function yourls_make_bookmarklet( $code ) {
  */
 function yourls_get_timestamp( $timestamp ) {
     $offset = yourls_get_time_offset();
-    $timestamp_offset = $timestamp + ($offset * 3600);
+    $timestamp_offset = (int)$timestamp + ($offset * 3600);
 
     return yourls_apply_filter( 'get_timestamp', $timestamp_offset, $timestamp, $offset );
 }
@@ -793,4 +796,3 @@ function yourls_get_date_format( $format ) {
 function yourls_get_time_format( $format ) {
     return yourls_apply_filter( 'get_time_format', (string)$format );
 }
-
diff --git a/includes/functions-html.php b/includes/functions-html.php
index 2ae620f..a619ffd 100644
--- a/includes/functions-html.php
+++ b/includes/functions-html.php
@@ -3,6 +3,7 @@
 /**
  * Display <h1> header and logo
  *
+ * @return void
  */
 function yourls_html_logo() {
 	yourls_do_action( 'pre_html_logo' );
@@ -22,6 +23,7 @@ function yourls_html_logo() {
  *
  * @param string $context Context of the page (stats, index, infos, ...)
  * @param string $title HTML title of the page
+ * @return void
  */
 function yourls_html_head( $context = 'index', $title = '' ) {
 
@@ -172,6 +174,7 @@ function yourls_html_footer($can_query = true) {
  *
  * @param string $url URL to prefill the input with
  * @param string $keyword Keyword to prefill the input with
+ * @return void
  */
 function yourls_html_addnew( $url = '', $keyword = '' ) {
     $pre = yourls_apply_filter( 'shunt_html_addnew', false, $url, $keyword );
@@ -205,7 +208,7 @@ function yourls_html_addnew( $url = '', $keyword = '' ) {
  * The $param array is defined in /admin/index.php, check the yourls_html_tfooter() call
  *
  * @param array $params Array of all required parameters
- * @return string Result
+ * @return void
  */
 function yourls_html_tfooter( $params = array() ) {
     // Manually extract all parameters from the array. We prefer doing it this way, over using extract(),
@@ -310,7 +313,7 @@ function yourls_html_tfooter( $params = array() ) {
 
 			<?php
 			// Remove empty keys from the $params array so it doesn't clutter the pagination links
-			$params = array_filter( $params, 'yourls_return_if_not_empty_string' ); // remove empty keys
+			$params = array_filter( $params, function($val){ return $val !== '';} ); // remove keys with empty values
 
 			if( isset( $search_text ) ) {
 				$params['search'] = $search_text;
@@ -384,9 +387,18 @@ function yourls_html_select( $name, $options, $selected = '', $display = false,
 	return $html;
 }
 
+
 /**
  * Display the Quick Share box
  *
+ * @param string $longurl          Long URL
+ * @param string $shorturl         Short URL
+ * @param string $title            Title
+ * @param string $text             Text to display
+ * @param string $shortlink_title  Optional replacement for 'Your short link'
+ * @param string $share_title      Optional replacement for 'Quick Share'
+ * @param bool   $hidden           Optional. Hide the box by default (with css "display:none")
+ * @return void
  */
 function yourls_share_box( $longurl, $shorturl, $title = '', $text='', $shortlink_title = '', $share_title = '', $hidden = false ) {
 	if ( $shortlink_title == '' )
@@ -460,6 +472,11 @@ function yourls_share_box( $longurl, $shorturl, $title = '', $text='', $shortlin
 /**
  * Die die die
  *
+ * @see https://www.youtube.com/watch?v=zSiKETBjARk
+ * @param string $message
+ * @param string $title
+ * @param int $header_code
+ * @return void
  */
 function yourls_die( $message = '', $title = '', $header_code = 200 ) {
     yourls_do_action( 'pre_yourls_die', $message, $title, $header_code );
@@ -522,7 +539,13 @@ RETURN;
 /**
  * Return an "Add" row for the main table
  *
- * @return string HTML of the edit row
+ * @param string $keyword     Keyword (short URL)
+ * @param string $url         URL (long URL)
+ * @param string $title       Title
+ * @param string $ip          IP
+ * @param string|int $clicks  Number of clicks
+ * @param string $timestamp   Timestamp
+ * @return string             HTML of the row
  */
 function yourls_table_add_row( $keyword, $url, $title, $ip, $clicks, $timestamp ) {
 	$keyword  = yourls_sanitize_keyword($keyword);
@@ -614,7 +637,7 @@ function yourls_table_add_row( $keyword, $url, $title, $ip, $clicks, $timestamp
 		),
 		'clicks' => array(
 			'template' => '%clicks%',
-			'clicks'   => yourls_number_format_i18n( $clicks, 0, '', '' ),
+			'clicks'   => yourls_number_format_i18n( $clicks, 0 ),
 		),
 		'actions' => array(
 			'template' => '%actions% <input type="hidden" id="keyword_%id%" value="%keyword%"/>',
@@ -641,6 +664,7 @@ function yourls_table_add_row( $keyword, $url, $title, $ip, $clicks, $timestamp
 /**
  * Echo the main table head
  *
+ * @return void
  */
 function yourls_table_head() {
 	$start = '<table id="main_table" class="tblSorter" cellpadding="0" cellspacing="1"><thead><tr>'."\n";
@@ -665,6 +689,7 @@ function yourls_table_head() {
 /**
  * Echo the tbody start tag
  *
+ * @return void
  */
 function yourls_table_tbody_start() {
 	echo yourls_apply_filter( 'table_tbody_start', '<tbody>' );
@@ -673,6 +698,7 @@ function yourls_table_tbody_start() {
 /**
  * Echo the tbody end tag
  *
+ * @return void
  */
 function yourls_table_tbody_end() {
 	echo yourls_apply_filter( 'table_tbody_end', '</tbody>' );
@@ -681,27 +707,36 @@ function yourls_table_tbody_end() {
 /**
  * Echo the table start tag
  *
+ * @return void
  */
 function yourls_table_end() {
 	echo yourls_apply_filter( 'table_end', '</table></main>' );
 }
 
+
+
 /**
  * Echo HTML tag for a link
  *
- */
-function yourls_html_link( $href, $title = '', $element = '' ) {
-	if( !$title )
-		$title = $href;
+ * @param string $href     URL to link to
+ * @param string $anchor   Anchor text
+ * @param string $element  Element id
+ * @return void
+*/
+function yourls_html_link( $href, $anchor = '', $element = '' ) {
+	if( !$anchor )
+		$anchor = $href;
 	if( $element )
 		$element = sprintf( 'id="%s"', yourls_esc_attr( $element ) );
-	$link = sprintf( '<a href="%s" %s>%s</a>', yourls_esc_url( $href ), $element, yourls_esc_html( $title ) );
+	$link = sprintf( '<a href="%s" %s>%s</a>', yourls_esc_url( $href ), $element, yourls_esc_html( $anchor ) );
 	echo yourls_apply_filter( 'html_link', $link );
 }
 
 /**
  * Display the login screen. Nothing past this point.
  *
+ * @param string $error_msg  Optional error message to display
+ * @return void
  */
 function yourls_login_screen( $error_msg = '' ) {
 	yourls_html_head( 'login' );
@@ -744,12 +779,13 @@ function yourls_login_screen( $error_msg = '' ) {
 	die();
 }
 
+
 /**
  * Display the admin menu
  *
+ * @return void
  */
 function yourls_html_menu() {
-
 	// Build menu links
 	if( defined( 'YOURLS_USER' ) ) {
 	    // Create a logout link with a nonce associated to fake user 'logout' : the user is not yet defined
@@ -828,6 +864,9 @@ function yourls_html_menu() {
 /**
  * Wrapper function to display admin notices
  *
+ * @param string $message Message to display
+ * @param string $style    Message style (default: 'notice')
+ * @return void
  */
 function yourls_add_notice( $message, $style = 'notice' ) {
 	// Escape single quotes in $message to avoid breaking the anonymous function
@@ -838,6 +877,9 @@ function yourls_add_notice( $message, $style = 'notice' ) {
 /**
  * Return a formatted notice
  *
+ * @param string $message  Message to display
+ * @param string $style    CSS class to use for the notice
+ * @return string          HTML of the notice
  */
 function yourls_notice_box( $message, $style = 'notice' ) {
 	return <<<HTML
@@ -848,13 +890,14 @@ HTML;
 }
 
 /**
- *  Display a page
+ * Display a page
  *
- *  Includes content of a PHP file from the YOURLS_PAGEDIR directory, as if it
- *  were a standard short URL (ie http://sho.rt/$page)
+ * Includes content of a PHP file from the YOURLS_PAGEDIR directory, as if it
+ * were a standard short URL (ie http://sho.rt/$page)
  *
- *  @since 1.0
- *  @param $page      PHP file to display
+ * @since 1.0
+ * @param string $page  PHP file to display
+ * @return void
  */
 function yourls_page( $page ) {
     if( !yourls_is_page($page)) {
@@ -873,6 +916,7 @@ function yourls_page( $page ) {
  * information for the page. Stolen from WP.
  *
  * @since 1.6
+ * @return void
  */
 function yourls_html_language_attributes() {
 	$attributes = array();
@@ -899,6 +943,7 @@ function yourls_html_language_attributes() {
  * Output translated strings used by the Javascript calendar
  *
  * @since 1.6
+ * @return void
  */
 function yourls_l10n_calendar_strings() {
 	echo "\n<script>\n";
@@ -919,8 +964,9 @@ function yourls_l10n_calendar_strings() {
  *
  * @since 1.7
  * @param string $compare_with Optional, YOURLS version to compare to
+ * @return void
  */
-function yourls_new_core_version_notice($compare_with = false) {
+function yourls_new_core_version_notice($compare_with = null) {
     $compare_with = $compare_with ?: YOURLS_VERSION;
 
 	$checks = yourls_get_option( 'core_version_checks' );
@@ -971,7 +1017,7 @@ function yourls_set_html_context($context) {
  * @return string
  */
 function yourls_get_html_context() {
-    yourls_get_db()->get_html_context();
+    return yourls_get_db()->get_html_context();
 }
 
 /**
diff --git a/includes/functions-http.php b/includes/functions-http.php
index 16d7e71..db60abc 100644
--- a/includes/functions-http.php
+++ b/includes/functions-http.php
@@ -24,6 +24,10 @@ use WpOrg\Requests\Requests;
  *
  * @since 1.7
  * @see yourls_http_request
+ * @param string $url     URL to request
+ * @param array $headers  HTTP headers to send
+ * @param array $data     GET data
+ * @param array $options  Options to pass to Requests
  * @return mixed Response object, or error string
  */
 function yourls_http_get( $url, $headers = array(), $data = array(), $options = array() ) {
@@ -35,6 +39,10 @@ function yourls_http_get( $url, $headers = array(), $data = array(), $options =
  *
  * @since 1.7
  * @see yourls_http_request
+ * @param string $url     URL to request
+ * @param array $headers  HTTP headers to send
+ * @param array $data     GET data
+ * @param array $options  Options to pass to Requests
  * @return mixed String (page body) or null if error
  */
 function yourls_http_get_body( $url, $headers = array(), $data = array(), $options = array() ) {
@@ -49,6 +57,10 @@ function yourls_http_get_body( $url, $headers = array(), $data = array(), $optio
  *
  * @since 1.7
  * @see yourls_http_request
+ * @param string $url     URL to request
+ * @param array $headers  HTTP headers to send
+ * @param array $data     POST data
+ * @param array $options  Options to pass to Requests
  * @return mixed Response object, or error string
  */
 function yourls_http_post( $url, $headers = array(), $data = array(), $options = array() ) {
@@ -62,6 +74,10 @@ function yourls_http_post( $url, $headers = array(), $data = array(), $options =
  *
  * @since 1.7
  * @see yourls_http_request
+ * @param string $url     URL to request
+ * @param array $headers  HTTP headers to send
+ * @param array $data     POST data
+ * @param array $options  Options to pass to Requests
  * @return mixed String (page body) or null if error
  */
 function yourls_http_post_body( $url, $headers = array(), $data = array(), $options = array() ) {
@@ -360,8 +376,8 @@ function yourls_check_core_version() {
  *  3) the object should not contain any other key
  *
  *  @since 1.7.7
- *  @param $json  JSON object to check
- *  @return bool  true if seems legit, false otherwise
+ *  @param object $json  JSON object to check
+ *  @return bool   true if seems legit, false otherwise
  */
 function yourls_validate_core_version_response($json) {
     return (
@@ -375,13 +391,14 @@ function yourls_validate_core_version_response($json) {
 
 /**
  * Get version number from Github zipball URL (last part of URL, really)
+ *
  * @since 1.8.3
  * @param string $zipurl eg 'https://api.github.com/repos/YOURLS/YOURLS/zipball/1.2.3'
  * @return string
  */
 function yourls_get_version_from_zipball_url($zipurl) {
     $version = '';
-    $parts = explode('/', parse_url(yourls_sanitize_url($zipurl), PHP_URL_PATH));
+    $parts = explode('/', parse_url(yourls_sanitize_url($zipurl), PHP_URL_PATH) ?? '');
     // expect at least 1 slash in path, return last part
     if( count($parts) > 1 ) {
         $version = end($parts);
@@ -391,11 +408,15 @@ function yourls_get_version_from_zipball_url($zipurl) {
 
 /**
  * Check if URL is from YOURLS/YOURLS repo on github
+ *
+ * @since 1.8.3
+ * @param string $url  URL to check
+ * @return bool
  */
 function yourls_is_valid_github_repo_url($url) {
     $url = yourls_sanitize_url($url);
     return (
-        join('.',array_slice(explode('.',parse_url($url, PHP_URL_HOST)), -2, 2)) === 'github.com'
+        join('.',array_slice(explode('.', parse_url($url, PHP_URL_HOST) ?? ''), -2, 2)) === 'github.com'
             // explodes on '.' (['api','github','com']) and keeps the last two elements
             // to make sure domain is either github.com or one of its subdomain (api.github.com for instance)
             // TODO: keep an eye on Github API to make sure it doesn't change some day to another domain (githubapi.com, ...)
@@ -406,6 +427,7 @@ function yourls_is_valid_github_repo_url($url) {
 
 /**
  * Check if object has only expected keys 'latest' and 'zipurl' containing strings
+ *
  * @since 1.8.3
  * @param object $json
  * @return bool
diff --git a/includes/functions-infos.php b/includes/functions-infos.php
index 8c10fd9..ed97203 100644
--- a/includes/functions-infos.php
+++ b/includes/functions-infos.php
@@ -3,8 +3,11 @@
 /**
  * Echoes an image tag of Google Charts map from sorted array of 'country_code' => 'number of visits' (sort by DESC)
  *
+ * @param array $countries  Array of 'country_code' => 'number of visits'
+ * @param string $id        Optional HTML element ID
+ * @return void
  */
-function yourls_stats_countries_map( $countries, $id = null ) {
+function yourls_stats_countries_map($countries, $id = null) {
 
 	yourls_do_action( 'pre_stats_countries_map' );
 
@@ -29,11 +32,17 @@ function yourls_stats_countries_map( $countries, $id = null ) {
 	echo yourls_apply_filter( 'stats_countries_map', $map, $countries, $options, $id );
 }
 
+
 /**
  * Echoes an image tag of Google Charts pie from sorted array of 'data' => 'value' (sort by DESC). Optional $limit = (integer) limit list of X first countries, sorted by most visits
  *
+ * @param array $data  Array of 'data' => 'value'
+ * @param int $limit   Optional limit list of X first countries
+ * @param $size        Optional size of the image
+ * @param $id          Optional HTML element ID
+ * @return void
  */
-function yourls_stats_pie( $data, $limit = 10, $size = '340x220', $id = null ) {
+function yourls_stats_pie($data, $limit = 10, $size = '340x220', $id = null) {
 
 	yourls_do_action( 'pre_stats_pie' );
 
@@ -80,11 +89,14 @@ function yourls_stats_pie( $data, $limit = 10, $size = '340x220', $id = null ) {
 	echo yourls_apply_filter( 'stats_pie', $pie, $data, $limit, $size, $options, $id );
 }
 
+
 /**
  * Build a list of all daily values between d1/m1/y1 to d2/m2/y2.
  *
+ * @param array $dates
+ * @return array[]  Array of arrays of days, months, years
  */
-function yourls_build_list_of_days( $dates ) {
+function yourls_build_list_of_days($dates) {
 	/* Say we have an array like:
 	$dates = array (
 		2009 => array (
@@ -153,13 +165,17 @@ function yourls_build_list_of_days( $dates ) {
 	);
 }
 
+
 /**
  * Echoes an image tag of Google Charts line graph from array of values (eg 'number of clicks').
  *
  * $legend1_list & legend2_list are values used for the 2 x-axis labels. $id is an HTML/JS id
  *
+ * @param array $values  Array of values (eg 'number of clicks')
+ * @param string $id     HTML element id
+ * @return void
  */
-function yourls_stats_line( $values, $id = null ) {
+function yourls_stats_line($values, $id = null) {
 
 	yourls_do_action( 'pre_stats_line' );
 
@@ -195,20 +211,27 @@ function yourls_stats_line( $values, $id = null ) {
 	echo yourls_apply_filter( 'stats_line', $lineChart, $values, $options, $id );
 }
 
+
 /**
- * Return the number of days in a month. From php.net, used if PHP built without calendar functions
+ * Return the number of days in a month. From php.net.
  *
+ * @param int $month
+ * @param int $year
+ * @return int
  */
-function yourls_days_in_month( $month, $year ) {
+function yourls_days_in_month($month, $year) {
 	// calculate number of days in a month
 	return $month == 2 ? ( $year % 4 ? 28 : ( $year % 100 ? 29 : ( $year % 400 ? 28 : 29 ) ) ) : ( ( $month - 1 ) % 7 % 2 ? 30 : 31 );
 }
 
+
 /**
  * Get max value from date array of 'Aug 12, 2012' = '1337'
  *
+ * @param array $list_of_days
+ * @return array
  */
-function yourls_stats_get_best_day( $list_of_days ) {
+function yourls_stats_get_best_day($list_of_days) {
 	$max = max( $list_of_days );
 	foreach( $list_of_days as $k=>$v ) {
 		if ( $v == $max )
@@ -219,8 +242,11 @@ function yourls_stats_get_best_day( $list_of_days ) {
 /**
  * Return domain of a URL
  *
+ * @param string $url
+ * @param bool $include_scheme
+ * @return string
  */
-function yourls_get_domain( $url, $include_scheme = false ) {
+function yourls_get_domain($url, $include_scheme = false) {
 	$parse = @parse_url( $url ); // Hiding ugly stuff coming from malformed referrer URLs
 
 	// Get host & scheme. Fall back to path if not found.
@@ -236,19 +262,24 @@ function yourls_get_domain( $url, $include_scheme = false ) {
 	return $host;
 }
 
+
 /**
  * Return favicon URL
  *
+ * @param string $url
+ * @return string
  */
-function yourls_get_favicon_url( $url ) {
+function yourls_get_favicon_url($url) {
 	return yourls_match_current_protocol( '//www.google.com/s2/favicons?domain=' . yourls_get_domain( $url, false ) );
 }
 
 /**
  * Scale array of data from 0 to 100 max
  *
+ * @param array $data
+ * @return array
  */
-function yourls_scale_data( $data ) {
+function yourls_scale_data($data ) {
 	$max = max( $data );
 	if( $max > 100 ) {
 		foreach( $data as $k=>$v ) {
@@ -258,11 +289,19 @@ function yourls_scale_data( $data ) {
 	return $data;
 }
 
+
 /**
- * Tweak granularity of array $array: keep only $grain values. This make less accurate but less messy graphs when too much values. See http://code.google.com/apis/chart/formats.html#granularity
+ * Tweak granularity of array $array: keep only $grain values.
  *
+ * This make less accurate but less messy graphs when too much values.
+ * See https://developers.google.com/chart/image/docs/gallery/line_charts?hl=en#data-granularity
+ *
+ * @param array $array
+ * @param int $grain
+ * @param bool $preserve_max
+ * @return array
  */
-function yourls_array_granularity( $array, $grain = 100, $preserve_max = true ) {
+function yourls_array_granularity($array, $grain = 100, $preserve_max = true) {
 	if ( count( $array ) > $grain ) {
 		$max = max( $array );
 		$step = intval( count( $array ) / $grain );
@@ -286,8 +325,10 @@ function yourls_array_granularity( $array, $grain = 100, $preserve_max = true )
 /**
  * Transform data array to data table for Google API
  *
+ * @param array $data
+ * @return string
  */
-function yourls_google_array_to_data_table( $data ){
+function yourls_google_array_to_data_table($data){
 	$str  = "var data = google.visualization.arrayToDataTable([\n";
 	foreach( $data as $label => $values ){
 		if( !is_array( $values ) ) {
@@ -310,8 +351,13 @@ function yourls_google_array_to_data_table( $data ){
 /**
  * Return javascript code that will display the Google Chart
  *
+ * @param string $graph_type
+ * @param string $data
+ * @param var $options
+ * @param string $id
+ * @return string
  */
-function yourls_google_viz_code( $graph_type, $data, $options, $id ) {
+function yourls_google_viz_code($graph_type, $data, $options, $id ) {
 	$function_name = 'yourls_graph' . $id;
 	$code  = "\n<script id=\"$function_name\" type=\"text/javascript\">\n";
 	$code .= "function $function_name() { \n";
@@ -336,4 +382,3 @@ function yourls_google_viz_code( $graph_type, $data, $options, $id ) {
 
 	return $code;
 }
-
diff --git a/includes/functions-install.php b/includes/functions-install.php
index ff32559..2ccd118 100644
--- a/includes/functions-install.php
+++ b/includes/functions-install.php
@@ -13,6 +13,7 @@ function yourls_check_PDO() {
 /**
  * Check if server has MySQL 5.0+
  *
+ * @return bool
  */
 function yourls_check_database_version() {
     return ( version_compare( '5.0', yourls_get_database_version() ) <= 0 );
@@ -40,6 +41,7 @@ function yourls_get_database_version() {
  * As of 1.8 we advertise YOURLS as being 7.4+ but it should work on 7.2 (although untested)
  * so we don't want to strictly enforce a limitation that may not be necessary.
  *
+ * @return bool
  */
 function yourls_check_php_version() {
     return version_compare( PHP_VERSION, '7.2.0', '>=' );
@@ -48,6 +50,7 @@ function yourls_check_php_version() {
 /**
  * Check if server is an Apache
  *
+ * @return bool
  */
 function yourls_is_apache() {
 	if( !array_key_exists( 'SERVER_SOFTWARE', $_SERVER ) )
@@ -61,6 +64,7 @@ function yourls_is_apache() {
 /**
  * Check if server is running IIS
  *
+ * @return bool
  */
 function yourls_is_iis() {
 	return ( array_key_exists( 'SERVER_SOFTWARE', $_SERVER ) ? ( strpos( $_SERVER['SERVER_SOFTWARE'], 'IIS' ) !== false ) : false );
@@ -70,6 +74,7 @@ function yourls_is_iis() {
 /**
  * Create .htaccess or web.config. Returns boolean
  *
+ * @return bool
  */
 function yourls_create_htaccess() {
 	$host = parse_url( yourls_get_yourls_site() );
@@ -319,6 +324,8 @@ function yourls_insert_sample_links() {
 /**
  * Toggle maintenance mode. Inspired from WP. Returns true for success, false otherwise
  *
+ * @param bool $maintenance  True to enable, false to disable
+ * @return bool              True on success, false on failure
  */
 function yourls_maintenance_mode( $maintenance = true ) {
 
@@ -342,4 +349,3 @@ function yourls_maintenance_mode( $maintenance = true ) {
 		return @unlink($file);
 	}
 }
-
diff --git a/includes/functions-kses.php b/includes/functions-kses.php
index fac0cf6..08a20a2 100644
--- a/includes/functions-kses.php
+++ b/includes/functions-kses.php
@@ -39,17 +39,17 @@
  * - $yourls_allowedentitynames is used internally in KSES functions to sanitize HTML entities
  * - $yourls_allowedprotocols is used in various parts of YOURLS, not just in KSES, albeit being defined here
  * Two globals are not defined and unused at this moment: $yourls_allowedtags_all and $yourls_allowedtags
- * The code for these vars is here and ready for any future use 
+ * The code for these vars is here and ready for any future use
  */
 
 // Populate after plugins have loaded to allow user defined values
 yourls_add_action( 'plugins_loaded', 'yourls_kses_init' );
- 
+
 /**
  * Init KSES globals if not already defined (by a plugin)
  *
  * @since 1.6
- *
+ * @return void
  */
 function yourls_kses_init() {
 	global $yourls_allowedentitynames, $yourls_allowedprotocols;
@@ -57,13 +57,13 @@ function yourls_kses_init() {
 	if( ! $yourls_allowedentitynames ) {
 		$yourls_allowedentitynames = yourls_apply_filter( 'kses_allowed_entities', yourls_kses_allowed_entities() );
 	}
-	
+
 	if( ! $yourls_allowedprotocols ) {
 		$yourls_allowedprotocols   = yourls_apply_filter( 'kses_allowed_protocols', yourls_kses_allowed_protocols() );
-	}	
+	}
 
 	/** See NOTE ABOUT GLOBALS **
-	
+
 	if( ! $yourls_allowedtags_all ) {
 		$yourls_allowedtags_all = yourls_kses_allowed_tags_all();
 		$yourls_allowedtags_all = array_map( '_yourls_add_global_attributes', $yourls_allowedtags_all );
@@ -72,7 +72,7 @@ function yourls_kses_init() {
 		// User defined: let's sanitize
 		$yourls_allowedtags_all = yourls_kses_array_lc( $yourls_allowedtags_all );
 	}
-	
+
 	if( ! $yourls_allowedtags ) {
 		$yourls_allowedtags = yourls_kses_allowed_tags();
 		$yourls_allowedtags = array_map( '_yourls_add_global_attributes', $yourls_allowedtags );
@@ -84,7 +84,7 @@ function yourls_kses_init() {
 
 	/**/
 }
- 
+
 /**
  * Kses global for all allowable HTML tags.
  *
@@ -411,7 +411,7 @@ function yourls_kses_allowed_tags_all() {
 		'var' => array(),
 	);
 }
- 
+
 /**
  * Kses global for default allowable HTML tags. TODO: trim down to necessary only.
  *
@@ -522,39 +522,39 @@ function yourls_kses_allowed_protocols() {
 		'feed:', 'feed://',
 		'mailto:',
 		'news:', 'nntp://',
-		
+
 		// Old school bearded geek
 		'gopher://', 'telnet://', 'finger://',
 		'nntp://', 'worldwind://',
-		
+
 		// Dev
 		'ssh://', 'svn://', 'svn+ssh://', 'git://', 'cvs://',
 		'apt:',
 		'market://', // Google Play
 		'view-source:',
-		
+
 		// P2P
 		'ed2k://', 'magnet:', 'udp://',
-		
+
 		// Streaming stuff
 		'mms://', 'lastfm://', 'spotify:', 'rtsp://',
 
 		// Text & voice
 		'aim:', 'facetime://', 'gtalk:', 'xmpp:',
-		'irc://', 'ircs://', 'mumble://', 
+		'irc://', 'ircs://', 'mumble://',
 		'callto:', 'skype:', 'sip:',
-		'teamspeak://', 'tel:', 'ventrilo://', 'xfire:', 
+		'teamspeak://', 'tel:', 'ventrilo://', 'xfire:',
 		'ymsgr:', 'tg://', 'whatsapp://',
 
 		// Misc
 		'steam:', 'steam://',
 		'bitcoin:',
 		'ldap://', 'ldaps://',
-		
+
 		// Purposedly removed for security
 		/*
 		'about:', 'chrome://', 'chrome-extension://',
-		'javascript:', 
+		'javascript:',
 		'data:',
 		*/
 	);
diff --git a/includes/functions-l10n.php b/includes/functions-l10n.php
index ead4c3c..d72c786 100644
--- a/includes/functions-l10n.php
+++ b/includes/functions-l10n.php
@@ -32,7 +32,7 @@ use POMO\Translations\NOOPTranslations;
  * @uses yourls_apply_filter() Calls 'get_locale' hook on locale value.
  * @uses $yourls_locale Gets the locale stored in the global.
  *
- * @return string The locale of the blog or from the 'get_locale' hook.
+ * @return string The locale of the YOURLS instance
  */
 function yourls_get_locale() {
 	global $yourls_locale;
@@ -116,8 +116,7 @@ function yourls__( $text, $domain = 'default' ) {
  * @see sprintf()
  * @since 1.6
  *
- * @param string $pattern Text to translate
- * @param string $arg1, $arg2... Optional: sprintf tokens, and translation domain
+ * @param mixed ...$pattern Text to translate, then $arg1: optional sprintf tokens, and $arg2: translation domain
  * @return string Translated text
  */
 function yourls_s( $pattern ) {
@@ -158,9 +157,8 @@ function yourls_s( $pattern ) {
  * @see sprintf()
  * @since 1.6
  *
- * @param string $pattern Text to translate
- * @param string $arg1, $arg2... Optional: sprintf tokens, and translation domain
- * @return string Translated text
+ * @param string ...$pattern Text to translate, then optional sprintf tokens, and optional translation domain
+ * @return void Translated text
  */
 function yourls_se( $pattern ) {
 	echo yourls_s( func_get_args() );
@@ -207,6 +205,7 @@ function yourls_esc_html__( $text, $domain = 'default' ) {
  *
  * @param string $text Text to translate
  * @param string $domain Optional. Domain to retrieve the translated text
+ * @return void
  */
 function yourls_e( $text, $domain = 'default' ) {
 	echo yourls_translate( $text, $domain );
@@ -221,6 +220,7 @@ function yourls_e( $text, $domain = 'default' ) {
  *
  * @param string $text Text to translate
  * @param string $domain Optional. Domain to retrieve the translated text
+ * @return void
  */
 function yourls_esc_attr_e( $text, $domain = 'default' ) {
 	echo yourls_esc_attr( yourls_translate( $text, $domain ) );
@@ -235,6 +235,7 @@ function yourls_esc_attr_e( $text, $domain = 'default' ) {
  *
  * @param string $text Text to translate
  * @param string $domain Optional. Domain to retrieve the translated text
+ * @return void
  */
 function yourls_esc_html_e( $text, $domain = 'default' ) {
 	echo yourls_esc_html( yourls_translate( $text, $domain ) );
@@ -269,7 +270,7 @@ function yourls_x( $text, $context, $domain = 'default' ) {
  * @param string $text Text to translate
  * @param string $context Context information for the translators
  * @param string $domain Optional. Domain to retrieve the translated text
- * @return string Translated context string
+ * @return void Echoes translated context string
  */
 function yourls_xe( $text, $context, $domain = 'default' ) {
 	echo yourls_x( $text, $context, $domain );
@@ -347,6 +348,12 @@ function yourls_n( $single, $plural, $number, $domain = 'default' ) {
  * @see yourls_n()
  * @see yourls_x()
  *
+ * @param string $single   The text that will be used if $number is 1
+ * @param string $plural   The text that will be used if $number is not 1
+ * @param int $number      The number to compare against to use either $single or $plural
+ * @param string $context  Context information for the translators
+ * @param string $domain   Optional. The domain identifier the text should be retrieved in
+ * @return string          Either $single or $plural translated text
  */
 function yourls_nx($single, $plural, $number, $context, $domain = 'default') {
 	$translations = yourls_get_translations_for_domain( $domain );
@@ -391,6 +398,12 @@ function yourls_n_noop( $singular, $plural, $domain = null ) {
  *
  * @since 1.6
  * @see yourls_n_noop()
+ *
+ * @param string $singular Single form to be i18ned
+ * @param string $plural   Plural form to be i18ned
+ * @param string $context  Context information for the translators
+ * @param string $domain   Optional. The domain identifier the text will be retrieved in
+ * @return array           array($singular, $plural)
  */
 function yourls_nx_noop( $singular, $plural, $context, $domain = null ) {
 	return array(
@@ -510,6 +523,8 @@ function yourls_load_default_textdomain() {
 
     if( !empty( $yourls_locale ) )
         return yourls_load_textdomain( 'default', YOURLS_LANG_DIR . "/$yourls_locale.mo" );
+
+    return false;
 }
 
 /**
@@ -548,6 +563,8 @@ function yourls_is_textdomain_loaded( $domain ) {
  * file and this function properly translates them back.
  *
  * @since 1.6
+ * @param string $name The role name
+ * @return string Translated role name
  */
 function yourls_translate_user_role( $name ) {
 	return yourls_translate_with_context( $name, 'User role' );
@@ -605,6 +622,9 @@ function yourls_number_format_i18n( $number, $decimals = 0 ) {
  * @return string                       The date, translated if locale specifies it.
  */
 function yourls_date_i18n( $dateformatstring, $timestamp = false ) {
+    /**
+     * @var YOURLS_Locale_Formats $yourls_locale_formats
+     */
 	global $yourls_locale_formats;
 	if( !isset( $yourls_locale_formats ) )
 		$yourls_locale_formats = new YOURLS_Locale_Formats();
@@ -743,6 +763,7 @@ class YOURLS_Locale_Formats {
 	 *
 	 * @since 1.6
 	 * @access private
+     * @return void
 	 */
 	function init() {
 		// The Weekdays
@@ -845,7 +866,7 @@ class YOURLS_Locale_Formats {
 	 * @since 1.6
 	 * @access public
 	 *
-	 * @param int $weekday_number 0 for Sunday through 6 Saturday
+	 * @param int|string $weekday_number 0 for Sunday through 6 Saturday
 	 * @return string Full translated weekday
 	 */
 	function get_weekday( $weekday_number ) {
@@ -945,6 +966,7 @@ class YOURLS_Locale_Formats {
 	 * @access private
 	 *
 	 * @since 1.6
+     * @return void
 	 */
 	function register_globals() {
 		$GLOBALS['weekday']         = $this->weekday;
diff --git a/includes/functions-links.php b/includes/functions-links.php
index 0775fc3..3bb994b 100644
--- a/includes/functions-links.php
+++ b/includes/functions-links.php
@@ -93,6 +93,8 @@ function yourls_add_query_arg() {
 /**
  * Navigates through an array and encodes the values to be used in a URL. Stolen from WP, used in yourls_add_query_arg()
  *
+ * @param array|string $value The array or string to be encoded.
+ * @return array|string
  */
 function yourls_urlencode_deep( $value ) {
     $value = is_array( $value ) ? array_map( 'yourls_urlencode_deep', $value ) : urlencode( $value );
@@ -147,6 +149,8 @@ function yourls_link( $keyword = '', $stats = false ) {
  *
  * This function does not make sure the keyword matches an actual short URL
  *
+ * @param  string $keyword  Short URL keyword
+ * @return string           Short URL stat link
  */
 function yourls_statlink( $keyword = '' ) {
     $link = yourls_link( $keyword, true );
@@ -156,6 +160,8 @@ function yourls_statlink( $keyword = '' ) {
 /**
  * Return admin link, with SSL preference if applicable.
  *
+ * @param string $page  Page name, eg "index.php"
+ * @return string
  */
 function yourls_admin_url( $page = '' ) {
     $admin = yourls_get_yourls_site() . '/admin/' . $page;
@@ -168,8 +174,11 @@ function yourls_admin_url( $page = '' ) {
 /**
  * Return YOURLS_SITE or URL under YOURLS setup, with SSL preference
  *
+ * @param bool $echo   Echo if true, or return if false
+ * @param string $url
+ * @return string
  */
-function yourls_site_url( $echo = true, $url = '' ) {
+function yourls_site_url($echo = true, $url = '' ) {
     $url = yourls_get_relative_url( $url );
     $url = trim( yourls_get_yourls_site() . '/' . $url, '/' );
 
@@ -255,7 +264,7 @@ function yourls_get_yourls_favicon_url( $echo = true ) {
     if( $custom ) {
         $favicon = yourls_site_url( false, YOURLS_USERURL . '/' . $custom );
     } else {
-        $favicon = yourls_site_url( false ) . '/images/favicon.ico';
+        $favicon = yourls_site_url( false ) . '/images/favicon.svg';
     }
 
     $favicon = yourls_apply_filter('get_favicon_url', $favicon);
diff --git a/includes/functions-options.php b/includes/functions-options.php
index 42e8810..f2728f4 100644
--- a/includes/functions-options.php
+++ b/includes/functions-options.php
@@ -36,6 +36,7 @@ function yourls_get_option( $option_name, $default = false ) {
  * a check for DB server reachability has been performed
  *
  * @since 1.4
+ * @return void
  */
 function yourls_get_all_options() {
     // Allow plugins to short-circuit all options. (Note: regular plugins are loaded after all options)
diff --git a/includes/functions-plugins.php b/includes/functions-plugins.php
index 6e64f94..dab4f96 100644
--- a/includes/functions-plugins.php
+++ b/includes/functions-plugins.php
@@ -55,7 +55,7 @@
  *          )
  *  )
  *
- * @var array
+ * @var array $yourls_filters
  */
 if ( !isset( $yourls_filters ) ) {
     $yourls_filters = [];
@@ -65,7 +65,7 @@ if ( !isset( $yourls_filters ) ) {
  * This global var will collect 'done' actions with the following structure:
  * $yourls_actions['hook'] => number of time this action was done
  *
- * @var array
+ * @var array $yourls_actions
  */
 if ( !isset( $yourls_actions ) ) {
     $yourls_actions = [];
@@ -79,7 +79,7 @@ if ( !isset( $yourls_actions ) ) {
  *
  * @link  https://docs.yourls.org/development/plugins.html
  * @param string   $hook           the name of the YOURLS element to be filtered or YOURLS action to be triggered
- * @param callback $function_name  the name of the function that is to be called.
+ * @param callable $function_name  the name of the function that is to be called.
  * @param int      $priority       optional. Used to specify the order in which the functions associated with a
  *                                 particular action are executed (default=10, lower=earlier execution, and functions
  *                                 with the same priority are executed in the order in which they were added to the
@@ -88,11 +88,12 @@ if ( !isset( $yourls_actions ) ) {
  *                                 provided).
  * @param string   $type
  * @global array   $yourls_filters Storage for all of the filters
+ * @return void
  */
 function yourls_add_filter( $hook, $function_name, $priority = 10, $accepted_args = NULL, $type = 'filter' ) {
     global $yourls_filters;
     // At this point, we cannot check if the function exists, as it may well be defined later (which is OK)
-    $id = yourls_filter_unique_id( $hook, $function_name, $priority );
+    $id = yourls_filter_unique_id($function_name);
 
     $yourls_filters[ $hook ][ $priority ][ $id ] = [
         'function'      => $function_name,
@@ -114,11 +115,12 @@ function yourls_add_filter( $hook, $function_name, $priority = 10, $accepted_arg
  *
  * @link  https://docs.yourls.org/development/plugins.html
  * @param string   $hook           The name of the action to which the $function_to_add is hooked.
- * @param callback $function_name  The name of the function you wish to be called.
+ * @param callable $function_name  The name of the function you wish to be called.
  * @param int      $priority       Optional. Used to specify the order in which the functions associated with a particular action
  *                                 are executed (default: 10). Lower numbers correspond with earlier execution, and functions
  *                                 with the same priority are executed in the order in which they were added to the action.
  * @param int      $accepted_args  Optional. The number of arguments the function accept (default 1).
+ * @return void
  */
 function yourls_add_action( $hook, $function_name, $priority = 10, $accepted_args = 1 ) {
 	yourls_add_filter( $hook, $function_name, $priority, $accepted_args, 'action' );
@@ -142,14 +144,11 @@ function yourls_add_action( $hook, $function_name, $priority = 10, $accepted_arg
  *     yourls_add_filter('my_hook_test', $my_callback_function);
  *
  * @link https://docs.yourls.org/development/hooks.html
- * @param  string       $hook            Hook to which the function is attached
- * @param  string|array $function        Used for creating unique id
- * @param  int|bool     $priority        Used in counting how many hooks were applied.  If === false and $function is an object reference,
- *                                       we return the unique id only if it already has one, false otherwise.
+ * @param  string|array|object $function  The callable used in a filter or action.
  * @return string  unique ID for usage as array key
  */
-function yourls_filter_unique_id( $hook, $function, $priority ) {
-    // If function then just skip all of the tests and not overwrite the following.
+function yourls_filter_unique_id($function) {
+    // If given a string (function name)
     if ( is_string( $function ) ) {
         return $function;
     }
@@ -167,20 +166,12 @@ function yourls_filter_unique_id( $hook, $function, $priority ) {
         return spl_object_hash( $function[0] ).$function[1];
     }
 
-    // Static Calling
-    if ( is_string( $function[0] ) ) {
-        return $function[0].'::'.$function[1];
-    }
-
-    /**
-     * There is no other possible case as of PHP 7.2-8.0 callables. Still, we're leaving the final
-     * `if` block (which could be remove to simply `return $function[0].'::'.$function[1]`) for readability
-     * and understanding the logic.
-     */
+    // Last case, static Calling : $function[0] is a string (Class Name) and $function[1] is a string (Method Name)
+    return $function[0].'::'.$function[1];
 }
 
 /**
- * Performs a filtering operation on a YOURLS element or event.
+ * Performs a filtering operation on a value or an event.
  *
  * Typical use:
  *
@@ -192,12 +183,12 @@ function yourls_filter_unique_id( $hook, $function, $priority ) {
  *		yourls_apply_filter( 'yourls_event' );
  *      (see yourls_do_action() )
  *
- * Returns an element which may have been filtered by a filter.
+ * Returns a value which may have been modified by a filter.
  *
  * @global array $yourls_filters storage for all of the filters
  * @param string $hook the name of the YOURLS element or action
  * @param mixed $value the value of the element before filtering
- * @param bool $is_action true if the function is called by yourls_do_action()
+ * @param true|mixed $is_action true if the function is called by yourls_do_action() - otherwise may be the second parameter of an arbitrary number of parameters
  * @return mixed
  */
 function yourls_apply_filter( $hook, $value = '', $is_action = false ) {
@@ -222,6 +213,7 @@ function yourls_apply_filter( $hook, $value = '', $is_action = false ) {
     reset( $yourls_filters[ $hook ] );
     do {
         foreach ( (array)current( $yourls_filters[ $hook ] ) as $the_ ) {
+            $_value = '';
             if ( !is_null($the_[ 'function' ]) ) {
                 $args[ 1 ] = $value;
                 $count = $the_[ 'accepted_args' ];
@@ -247,6 +239,7 @@ function yourls_apply_filter( $hook, $value = '', $is_action = false ) {
  *
  * @param string $hook the name of the YOURLS action
  * @param mixed $arg action arguments
+ * @return void
  */
 function yourls_do_action( $hook, $arg = '' ) {
     global $yourls_actions, $yourls_filters;
@@ -306,6 +299,7 @@ function yourls_did_action( $hook ) {
  * @param  string $type Either 'action' or 'filter'
  * @param  string $hook The hook name, eg 'plugins_loaded'
  * @param  mixed  $args Variable-length argument lists that were passed to the action or filter
+ * @return void
  */
 function yourls_call_all_hooks($type, $hook, ...$args) {
     global $yourls_filters;
@@ -340,14 +334,14 @@ function yourls_call_all_hooks($type, $hook, ...$args) {
  *
  * @global array $yourls_filters storage for all of the filters
  * @param string $hook The filter hook to which the function to be removed is hooked.
- * @param callback $function_to_remove The name of the function which should be removed.
+ * @param callable $function_to_remove The name of the function which should be removed.
  * @param int $priority optional. The priority of the function (default: 10).
  * @return bool Whether the function was registered as a filter before it was removed.
  */
 function yourls_remove_filter( $hook, $function_to_remove, $priority = 10 ) {
     global $yourls_filters;
 
-    $function_to_remove = yourls_filter_unique_id( $hook, $function_to_remove, $priority );
+    $function_to_remove = yourls_filter_unique_id($function_to_remove);
 
     $remove = isset( $yourls_filters[ $hook ][ $priority ][ $function_to_remove ] );
 
@@ -435,15 +429,14 @@ function yourls_get_filters($hook) {
 function yourls_get_actions($hook) {
     return yourls_get_filters($hook);
 }
-
 /**
  * Check if any filter has been registered for a hook.
  *
  * @since 1.5
+ * @global array         $yourls_filters    storage for all of the filters
  * @param string         $hook              The name of the filter hook.
  * @param callable|false $function_to_check optional. If specified, return the priority of that function on this hook or false if not attached.
  * @return int|bool Optionally returns the priority on that hook for the specified function.
- * @global array         $yourls_filters    storage for all of the filters
  */
 function yourls_has_filter( $hook, $function_to_check = false ) {
     global $yourls_filters;
@@ -453,7 +446,7 @@ function yourls_has_filter( $hook, $function_to_check = false ) {
         return $has;
     }
 
-    if ( !$idx = yourls_filter_unique_id( $hook, $function_to_check, false ) ) {
+    if ( !$idx = yourls_filter_unique_id($function_to_check) ) {
         return false;
     }
 
@@ -465,6 +458,7 @@ function yourls_has_filter( $hook, $function_to_check = false ) {
     return false;
 }
 
+
 /**
  * Check if any action has been registered for a hook.
  *
@@ -786,6 +780,7 @@ function yourls_list_plugin_admin_pages() {
  * @param string   $slug
  * @param string   $title
  * @param callable $function
+ * @return void
  */
 function yourls_register_plugin_page( $slug, $title, $function ) {
     yourls_get_db()->add_plugin_page( $slug, $title, $function );
@@ -796,6 +791,7 @@ function yourls_register_plugin_page( $slug, $title, $function ) {
  *
  * @since 1.5
  * @param string $plugin_page
+ * @return void
  */
 function yourls_plugin_admin_page( $plugin_page ) {
     // Check the plugin page is actually registered
@@ -866,6 +862,7 @@ function yourls_plugins_sort_callback( $plugin_a, $plugin_b ) {
  *
  * @codeCoverageIgnore
  * @since 1.5.1
+ * @return void
  */
 function yourls_shutdown() {
     yourls_do_action( 'shutdown' );
diff --git a/includes/functions-shorturls.php b/includes/functions-shorturls.php
index 3437bcc..93dffb7 100644
--- a/includes/functions-shorturls.php
+++ b/includes/functions-shorturls.php
@@ -84,6 +84,7 @@ function yourls_add_new_link( $url, $keyword = '', $title = '' ) {
             yourls_trim_long_string($url), preg_replace('!https?://!', '',  yourls_get_yourls_site()) . '/'. $url_exists->keyword );
         $return['title']    = $url_exists->title;
         $return['shorturl'] = yourls_link($url_exists->keyword);
+        $return['errorCode'] = $return['statusCode'] = '400'; // 400 Bad Request
 
         return yourls_apply_filter( 'add_new_link_already_stored_filter', $return, $url, $keyword, $title );
     }
@@ -168,11 +169,6 @@ function yourls_add_new_link( $url, $keyword = '', $title = '' ) {
  * @return string    Acceptable charset for short URLS keywords
  */
 function yourls_get_shorturl_charset() {
-    static $charset = null;
-    if ( $charset !== null ) {
-        return $charset;
-    }
-
     if ( defined( 'YOURLS_URL_CONVERT' ) && in_array( YOURLS_URL_CONVERT, [ 62, 64 ] ) ) {
         $charset = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
     }
@@ -233,6 +229,8 @@ function yourls_keyword_is_reserved( $keyword ) {
 /**
  * Delete a link in the DB
  *
+ * @param  string $keyword   Short URL keyword
+ * @return int               Number of links deleted
  */
 function yourls_delete_link_by_keyword( $keyword ) {
     // Allow plugins to short-circuit the whole function
@@ -251,8 +249,12 @@ function yourls_delete_link_by_keyword( $keyword ) {
 /**
  * SQL query to insert a new link in the DB. Returns boolean for success or failure of the inserting
  *
+ * @param string $url
+ * @param string $keyword
+ * @param string $title
+ * @return bool true if insert succeeded, false if failed
  */
-function yourls_insert_link_in_db( $url, $keyword, $title = '' ) {
+function yourls_insert_link_in_db($url, $keyword, $title = '' ) {
     $url       = yourls_sanitize_url($url);
     $keyword   = yourls_sanitize_keyword($keyword);
     $title     = yourls_sanitize_title($title);
@@ -304,8 +306,13 @@ function yourls_long_url_exists( $url ) {
 /**
  * Edit a link
  *
+ * @param string $url
+ * @param string $keyword
+ * @param string $newkeyword
+ * @param string $title
+ * @return array Result of the edit and link information if successful
  */
-function yourls_edit_link( $url, $keyword, $newkeyword='', $title='' ) {
+function yourls_edit_link($url, $keyword, $newkeyword='', $title='' ) {
     // Allow plugins to short-circuit the whole function
     $pre = yourls_apply_filter( 'shunt_edit_link', null, $keyword, $url, $keyword, $newkeyword, $title );
     if ( null !== $pre )
@@ -375,6 +382,9 @@ function yourls_edit_link( $url, $keyword, $newkeyword='', $title='' ) {
 /**
  * Update a title link (no checks for duplicates etc..)
  *
+ * @param string $keyword
+ * @param string $title
+ * @return int number of rows updated
  */
 function yourls_edit_link_title( $keyword, $title ) {
     // Allow plugins to short-circuit the whole function
@@ -487,10 +497,14 @@ function yourls_get_keyword_infos( $keyword, $use_cache = true ) {
 }
 
 /**
- * Return (string) selected information associated with a keyword. Optional $notfound = string default message if nothing found
+ * Return information associated with a keyword (eg clicks, URL, title...). Optional $notfound = string default message if nothing found
  *
+ * @param string $keyword          Short URL keyword
+ * @param string $field            Field to return (eg 'url', 'title', 'ip', 'clicks', 'timestamp', 'keyword')
+ * @param false|string $notfound   Optional string to return if keyword not found
+ * @return mixed|string
  */
-function yourls_get_keyword_info( $keyword, $field, $notfound = false ) {
+function yourls_get_keyword_info($keyword, $field, $notfound = false ) {
 
     // Allow plugins to short-circuit the whole function
     $pre = yourls_apply_filter( 'shunt_get_keyword_info', false, $keyword, $field, $notfound );
@@ -510,6 +524,9 @@ function yourls_get_keyword_info( $keyword, $field, $notfound = false ) {
 /**
  * Return title associated with keyword. Optional $notfound = string default message if nothing found
  *
+ * @param string $keyword          Short URL keyword
+ * @param false|string $notfound   Optional string to return if keyword not found
+ * @return mixed|string
  */
 function yourls_get_keyword_title( $keyword, $notfound = false ) {
     return yourls_get_keyword_info( $keyword, 'title', $notfound );
@@ -518,6 +535,9 @@ function yourls_get_keyword_title( $keyword, $notfound = false ) {
 /**
  * Return long URL associated with keyword. Optional $notfound = string default message if nothing found
  *
+ * @param string $keyword          Short URL keyword
+ * @param false|string $notfound   Optional string to return if keyword not found
+ * @return mixed|string
  */
 function yourls_get_keyword_longurl( $keyword, $notfound = false ) {
     return yourls_get_keyword_info( $keyword, 'url', $notfound );
@@ -526,6 +546,9 @@ function yourls_get_keyword_longurl( $keyword, $notfound = false ) {
 /**
  * Return number of clicks on a keyword. Optional $notfound = string default message if nothing found
  *
+ * @param string $keyword          Short URL keyword
+ * @param false|string $notfound   Optional string to return if keyword not found
+ * @return mixed|string
  */
 function yourls_get_keyword_clicks( $keyword, $notfound = false ) {
     return yourls_get_keyword_info( $keyword, 'clicks', $notfound );
@@ -534,6 +557,9 @@ function yourls_get_keyword_clicks( $keyword, $notfound = false ) {
 /**
  * Return IP that added a keyword. Optional $notfound = string default message if nothing found
  *
+ * @param string $keyword          Short URL keyword
+ * @param false|string $notfound   Optional string to return if keyword not found
+ * @return mixed|string
  */
 function yourls_get_keyword_IP( $keyword, $notfound = false ) {
     return yourls_get_keyword_info( $keyword, 'ip', $notfound );
@@ -542,6 +568,9 @@ function yourls_get_keyword_IP( $keyword, $notfound = false ) {
 /**
  * Return timestamp associated with a keyword. Optional $notfound = string default message if nothing found
  *
+ * @param string $keyword          Short URL keyword
+ * @param false|string $notfound   Optional string to return if keyword not found
+ * @return mixed|string
  */
 function yourls_get_keyword_timestamp( $keyword, $notfound = false ) {
     return yourls_get_keyword_info( $keyword, 'timestamp', $notfound );
diff --git a/includes/functions-upgrade.php b/includes/functions-upgrade.php
index 2de8e0e..4b48145 100644
--- a/includes/functions-upgrade.php
+++ b/includes/functions-upgrade.php
@@ -7,8 +7,14 @@
  * rather than using the YOURLS version number, eg yourls_update_to_18(). This is to allow having
  * multiple SQL update during the dev cycle of the same Y0URLS version
  *
+ * @param string|int $step
+ * @param string $oldver
+ * @param string $newver
+ * @param string|int $oldsql
+ * @param string|int $newsql
+ * @return void
  */
-function yourls_upgrade( $step, $oldver, $newver, $oldsql, $newsql ) {
+function yourls_upgrade($step, $oldver, $newver, $oldsql, $newsql ) {
 
     /**
      *  Sanitize input. Two notes :
@@ -206,8 +212,8 @@ function yourls_upgrade_to_143( ) {
  */
 function yourls_upgrade_to_141( ) {
 	// Kill old cookies from 1.3 and prior
-	setcookie('yourls_username', null, time() - 3600 );
-	setcookie('yourls_password', null, time() - 3600 );
+	setcookie('yourls_username', '', time() - 3600 );
+	setcookie('yourls_password', '', time() - 3600 );
 	// alter table URL
 	yourls_alter_url_table_to_141();
 	// recreate the htaccess file if needed
@@ -445,4 +451,3 @@ function yourls_clean_htaccess_for_14() {
 
 	return $result;
 }
-
diff --git a/includes/functions.php b/includes/functions.php
index 55860ea..2f6725c 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -6,7 +6,8 @@
 
 /**
  * Make an optimized regexp pattern from a string of characters
- * @param $string
+ *
+ * @param string $string
  * @return string
  */
 function yourls_make_regexp_pattern( $string ) {
@@ -16,7 +17,8 @@ function yourls_make_regexp_pattern( $string ) {
 }
 
 /**
- * Function: Get client IP Address. Returns a DB safe string.
+ * Get client IP Address. Returns a DB safe string.
+ *
  * @return string
  */
 function yourls_get_IP() {
@@ -52,17 +54,17 @@ function yourls_get_next_decimal() {
  * Update id for next link with no custom keyword
  *
  * Note: this function relies upon yourls_update_option(), which will return either true or false
- * depending if there has been an actual MySQL query updating the DB.
+ * depending upon if there has been an actual MySQL query updating the DB.
  * In other words, this function may return false yet this would not mean it has functionally failed
- * In other words I'm not sure we really need this function to return something :face_with_eyes_looking_up:
+ * In other words I'm not sure if we really need this function to return something :face_with_eyes_looking_up:
  * See issue 2621 for more on this.
  *
  * @since 1.0
- * @param integer $int     id for next link
- * @return bool            true or false depending on if there has been an actual MySQL query. See note above.
+ * @param integer $int id for next link
+ * @return bool        true or false depending on if there has been an actual MySQL query. See note above.
  */
-function yourls_update_next_decimal( $int = '' ) {
-	$int = ( $int == '' ) ? yourls_get_next_decimal() + 1 : (int)$int ;
+function yourls_update_next_decimal( $int = 0 ) {
+	$int = ( $int == 0 ) ? yourls_get_next_decimal() + 1 : (int)$int ;
 	$update = yourls_update_option( 'next_id', $int );
 	yourls_do_action( 'update_next_decimal', $int, $update );
 	return $update;
@@ -70,18 +72,20 @@ function yourls_update_next_decimal( $int = '' ) {
 
 /**
  * Return XML output.
+ *
  * @param array $array
  * @return string
  */
 function yourls_xml_encode( $array ) {
-    return (\Spatie\ArrayToXml\ArrayToXml::convert($array));
+    return (\Spatie\ArrayToXml\ArrayToXml::convert($array, '', true, 'UTF-8'));
 }
 
 /**
  * Update click count on a short URL. Return 0/1 for error/success.
+ *
  * @param string $keyword
- * @param bool   $clicks
- * @return mixed|string
+ * @param false|int $clicks
+ * @return int 0 or 1 for error/success
  */
 function yourls_update_clicks( $keyword, $clicks = false ) {
 	// Allow plugins to short-circuit the whole function
@@ -112,11 +116,16 @@ function yourls_update_clicks( $keyword, $clicks = false ) {
 	return $result;
 }
 
+
 /**
  * Return array of stats. (string)$filter is 'bottom', 'last', 'rand' or 'top'. (int)$limit is the number of links to return
  *
+ * @param string $filter  'bottom', 'last', 'rand' or 'top'
+ * @param int $limit      Number of links to return
+ * @param int $start      Offset to start from
+ * @return array          Array of links
  */
-function yourls_get_stats( $filter = 'top', $limit = 10, $start = 0 ) {
+function yourls_get_stats($filter = 'top', $limit = 10, $start = 0) {
 	switch( $filter ) {
 		case 'bottom':
 			$sort_by    = '`clicks`';
@@ -175,7 +184,7 @@ function yourls_get_stats( $filter = 'top', $limit = 10, $start = 0 ) {
  *   $where['sql'] will concatenate SQL clauses: $where['sql'] = ' AND something = :value AND otherthing < :othervalue';
  *   $where['binds'] will hold the (name => value) placeholder pairs: $where['binds'] = array('value' => $value, 'othervalue' => $value2)
  *
- * @param  $where array  See comment above
+ * @param  array $where See comment above
  * @return array
  */
 function yourls_get_db_stats( $where = [ 'sql' => '', 'binds' => [] ] ) {
@@ -190,6 +199,7 @@ function yourls_get_db_stats( $where = [ 'sql' => '', 'binds' => [] ] ) {
 /**
  * Returns a sanitized a user agent string. Given what I found on http://www.user-agents.org/ it should be OK.
  *
+ * @return string
  */
 function yourls_get_user_agent() {
     $ua = '-';
@@ -260,6 +270,7 @@ function yourls_redirect( $location, $code = 301 ) {
  * @since  1.7.3
  * @param  string $url
  * @param  string $keyword
+ * @return void
  */
 function yourls_redirect_shorturl($url, $keyword) {
     yourls_do_action( 'redirect_shorturl', $url, $keyword );
@@ -364,6 +375,7 @@ function yourls_status_header( $code = 200 ) {
  *
  * @param string $location
  * @param bool   $dontwait
+ * @return void
  */
 function yourls_redirect_javascript( $location, $dontwait = true ) {
     yourls_do_action( 'pre_redirect_javascript', $location, $dontwait );
@@ -385,6 +397,7 @@ REDIR;
 
 /**
  * Return an HTTP status code
+ *
  * @param int $code
  * @return string
  */
@@ -448,7 +461,7 @@ function yourls_get_HTTP_status( $code ) {
 		510 => 'Not Extended'
     ];
 
-    return isset( $headers_desc[ $code ] ) ? $headers_desc[ $code ] : '';
+    return $headers_desc[$code] ?? '';
 }
 
 /**
@@ -496,6 +509,7 @@ function yourls_log_redirect( $keyword ) {
 /**
  * Check if we want to not log redirects (for stats)
  *
+ * @return bool
  */
 function yourls_do_log_redirect() {
 	return ( !defined( 'YOURLS_NOSTATS' ) || YOURLS_NOSTATS != true );
@@ -503,6 +517,7 @@ function yourls_do_log_redirect() {
 
 /**
  * Check if an upgrade is needed
+ *
  * @return bool
  */
 function yourls_upgrade_is_needed() {
@@ -522,6 +537,7 @@ function yourls_upgrade_is_needed() {
 
 /**
  * Get current version & db version as stored in the options DB. Prior to 1.4 there's no option table.
+ *
  * @return array
  */
 function yourls_get_current_version_from_sql() {
@@ -542,6 +558,7 @@ function yourls_get_current_version_from_sql() {
 /**
  * Determine if the current page is private
  *
+ * @return bool
  */
 function yourls_is_private() {
     $private = defined( 'YOURLS_PRIVATE' ) && YOURLS_PRIVATE;
@@ -566,6 +583,7 @@ function yourls_is_private() {
 
 /**
  * Allow several short URLs for the same long URL ?
+ *
  * @return bool
  */
 function yourls_allow_duplicate_longurls() {
@@ -579,6 +597,7 @@ function yourls_allow_duplicate_longurls() {
 
 /**
  * Check if an IP shortens URL too fast to prevent DB flood. Return true, or die.
+ *
  * @param string $ip
  * @return bool|mixed|string
  */
@@ -735,6 +754,7 @@ function yourls_rnd_string ( $length = 5, $type = 0, $charlist = '' ) {
 
 /**
  * Check if we're in API mode.
+ *
  * @return bool
  */
 function yourls_is_API() {
@@ -743,6 +763,7 @@ function yourls_is_API() {
 
 /**
  * Check if we're in Ajax mode.
+ *
  * @return bool
  */
 function yourls_is_Ajax() {
@@ -751,6 +772,7 @@ function yourls_is_Ajax() {
 
 /**
  * Check if we're in GO mode (yourls-go.php).
+ *
  * @return bool
  */
 function yourls_is_GO() {
@@ -760,14 +782,16 @@ function yourls_is_GO() {
 /**
  * Check if we're displaying stats infos (yourls-infos.php). Returns bool
  *
+ * @return bool
  */
 function yourls_is_infos() {
     return (bool)yourls_apply_filter( 'is_infos', defined( 'YOURLS_INFOS' ) && YOURLS_INFOS );
 }
 
 /**
- * Check if we're in the admin area. Returns bool
+ * Check if we're in the admin area. Returns bool. Does not relate with user rights.
  *
+ * @return bool
  */
 function yourls_is_admin() {
     return (bool)yourls_apply_filter( 'is_admin', defined( 'YOURLS_ADMIN' ) && YOURLS_ADMIN );
@@ -776,6 +800,7 @@ function yourls_is_admin() {
 /**
  * Check if the server seems to be running on Windows. Not exactly sure how reliable this is.
  *
+ * @return bool
  */
 function yourls_is_windows() {
 	return defined( 'DIRECTORY_SEPARATOR' ) && DIRECTORY_SEPARATOR == '\\';
@@ -783,6 +808,7 @@ function yourls_is_windows() {
 
 /**
  * Check if SSL is required.
+ *
  * @return bool
  */
 function yourls_needs_ssl() {
@@ -791,6 +817,7 @@ function yourls_needs_ssl() {
 
 /**
  * Check if SSL is used. Stolen from WP.
+ *
  * @return bool
  */
 function yourls_is_ssl() {
@@ -902,6 +929,7 @@ function yourls_get_remote_title( $url ) {
 
 /**
  * Quick UA check for mobile devices.
+ *
  * @return bool
  */
 function yourls_is_mobile_device() {
@@ -935,7 +963,7 @@ function yourls_is_mobile_device() {
  * @param string $uri           Optional, page requested (default to $_SERVER['REQUEST_URI'] eg '/yourls/abcd' )
  * @return string               request relative to YOURLS base (eg 'abdc')
  */
-function yourls_get_request($yourls_site = false, $uri = false) {
+function yourls_get_request($yourls_site = '', $uri = '') {
     // Allow plugins to short-circuit the whole function
     $pre = yourls_apply_filter( 'shunt_get_request', false );
     if ( false !== $pre ) {
@@ -945,10 +973,10 @@ function yourls_get_request($yourls_site = false, $uri = false) {
     yourls_do_action( 'pre_get_request', $yourls_site, $uri );
 
     // Default values
-    if ( false === $yourls_site ) {
+    if ( '' === $yourls_site ) {
         $yourls_site = yourls_get_yourls_site();
     }
-    if ( false === $uri ) {
+    if ( '' === $uri ) {
         $uri = $_SERVER[ 'REQUEST_URI' ];
     }
 
@@ -986,6 +1014,7 @@ function yourls_get_request($yourls_site = false, $uri = false) {
 /**
  * Fix $_SERVER['REQUEST_URI'] variable for various setups. Stolen from WP.
  *
+ * @return void
  */
 function yourls_fix_request_uri() {
 
@@ -1033,19 +1062,21 @@ function yourls_fix_request_uri() {
 /**
  * Check for maintenance mode. If yes, die. See yourls_maintenance_mode(). Stolen from WP.
  *
+ * @return void
  */
 function yourls_check_maintenance_mode() {
-
 	$file = YOURLS_ABSPATH . '/.maintenance' ;
-	if ( !file_exists( $file ) || yourls_is_upgrading() || yourls_is_installing() )
-		return;
+
+    if ( !file_exists( $file ) || yourls_is_upgrading() || yourls_is_installing() ) {
+        return;
+    }
 
 	global $maintenance_start;
-
 	include_once( $file );
 	// If the $maintenance_start timestamp is older than 10 minutes, don't die.
-	if ( ( time() - $maintenance_start ) >= 600 )
-		return;
+	if ( ( time() - $maintenance_start ) >= 600 ) {
+        return;
+    }
 
 	// Use any /user/maintenance.php file
 	if( file_exists( YOURLS_USERDIR.'/maintenance.php' ) ) {
@@ -1053,29 +1084,11 @@ function yourls_check_maintenance_mode() {
 		die();
 	}
 
-	// https://www.youtube.com/watch?v=Xw-m4jEY-Ns
+    // Or use the default messages
 	$title   = yourls__( 'Service temporarily unavailable' );
 	$message = yourls__( 'Our service is currently undergoing scheduled maintenance.' ) . "</p>\n<p>" .
 	yourls__( 'Things should not last very long, thank you for your patience and please excuse the inconvenience' );
 	yourls_die( $message, $title , 503 );
-
-}
-
-/**
- * Return current admin page, or null if not an admin page
- *
- * @return mixed string if admin page, null if not an admin page
- * @since 1.6
- */
-function yourls_current_admin_page() {
-	if( yourls_is_admin() ) {
-		$current = substr( yourls_get_request(), 6 );
-		if( $current === false )
-			$current = 'index.php'; // if current page is http://sho.rt/admin/ instead of http://sho.rt/admin/index.php
-
-		return $current;
-	}
-	return null;
 }
 
 /**
@@ -1155,7 +1168,7 @@ function yourls_get_relative_url( $url, $strict = true ) {
 }
 
 /**
- * Marks a function as deprecated and informs when it has been used. Stolen from WP.
+ * Marks a function as deprecated and informs that it has been used. Stolen from WP.
  *
  * There is a hook deprecated_function that will be called that can be used
  * to get the backtrace up to what file and function called the deprecated
@@ -1174,6 +1187,7 @@ function yourls_get_relative_url( $url, $strict = true ) {
  * @param string $function The function that was called
  * @param string $version The version of WordPress that deprecated the function
  * @param string $replacement Optional. The function that should have been called
+ * @return void
  */
 function yourls_deprecated_function( $function, $version, $replacement = null ) {
 
@@ -1188,19 +1202,6 @@ function yourls_deprecated_function( $function, $version, $replacement = null )
 	}
 }
 
-/**
- * Return the value if not an empty string
- *
- * Used with array_filter(), to remove empty keys but not keys with value 0 or false
- *
- * @since 1.6
- * @param mixed $val Value to test against ''
- * @return bool True if not an empty string
- */
-function yourls_return_if_not_empty_string( $val ) {
-    return ( $val !== '' );
-}
-
 /**
  * Explode a URL in an array of ( 'protocol' , 'slashes if any', 'rest of the URL' )
  *
@@ -1243,14 +1244,14 @@ function yourls_get_protocol_slashes_and_rest( $url, $array = [ 'protocol', 'sla
 }
 
 /**
- * Set URL scheme (to HTTP or HTTPS)
+ * Set URL scheme (HTTP or HTTPS) to a URL
  *
  * @since 1.7.1
  * @param string $url    URL
  * @param string $scheme scheme, either 'http' or 'https'
  * @return string URL with chosen scheme
  */
-function yourls_set_url_scheme( $url, $scheme = false ) {
+function yourls_set_url_scheme( $url, $scheme = '' ) {
     if ( in_array( $scheme, [ 'http', 'https' ] ) ) {
         $url = preg_replace( '!^[a-zA-Z0-9+.-]+://!', $scheme.'://', $url );
     }
@@ -1260,10 +1261,11 @@ function yourls_set_url_scheme( $url, $scheme = false ) {
 /**
  * Tell if there is a new YOURLS version
  *
- * This function checks, if needed, if there's a new version of YOURLS and, if applicable, display
+ * This function checks, if needed, if there's a new version of YOURLS and, if applicable, displays
  * an update notice.
  *
  * @since 1.7.3
+ * @return void
  */
 function yourls_tell_if_new_version() {
     yourls_debug_log( 'Check for new version: '.( yourls_maybe_check_core_version() ? 'yes' : 'no' ) );
diff --git a/includes/version.php b/includes/version.php
index 2012bf0..48a95e5 100644
--- a/includes/version.php
+++ b/includes/version.php
@@ -9,7 +9,7 @@
  *    MAJOR.MINOR.PATCH-SOMETHING (1.8.1-donotuse)
  *
  */
-define( 'YOURLS_VERSION', '1.9' );
+define( 'YOURLS_VERSION', '1.9.1' );
 
 /**
  * YOURLS DB version. Increments when changes are made to the DB schema, to trigger a DB update