Fix custom URL for admin area and login page (#1324)

Custom URLs for admin area and login page are not handled properly
The path locations should not be hard-coded for neutral paths, referrer and slugs.
WP only provides access to the full URL with site included so the checks must be adapted.
Create a new function `qtranxf_get_admin_base` to.
Fix wrong check for neutral path from login page.

src/language_detect.php

@@ -29,10 +29,11 @@ function qtranxf_detect_language( array &$url_info ) {
     if ( isset( $_SERVER['HTTP_REFERER'] ) && $parse_referrer ) {
         $http_referer             = $_SERVER['HTTP_REFERER'];
         $url_info['http_referer'] = $http_referer;
-
         // if needed, detect front- vs back-end
         $parse_referrer_language = true;
-        if ( strpos( $http_referer, '/wp-admin' ) !== false ) {
+        // Attention: the referrer can be a full or partial path.
+        // TODO: improve check if admin base is found at other positions in URL
+        if ( strpos( $http_referer, '/' . qtranxf_get_admin_base() ) !== false ) {
             $url_info['referer_admin'] = true;
             if ( ! isset( $url_info['doing_front_end'] ) ) {
                 $url_info['doing_front_end'] = false;

src/modules/slugs/slugs.php

@@ -326,10 +326,11 @@ public function query_vars( $query_vars ) {
             }
 
             // If req_uri is empty or if it is a request for ourself, unset error.
-            if ( empty( $request ) || $req_uri == $self || strpos( $_SERVER['PHP_SELF'], 'wp-admin/' ) !== false ) {
+            // TODO: improve string check if admin base is found at other positions in URL.
+            if ( empty( $request ) || $req_uri == $self || strpos( $_SERVER['PHP_SELF'], qtranxf_get_admin_base() . '/' ) !== false ) {
                 unset( $_GET['error'] );
                 unset( $error );
-                if ( isset( $perma_query_vars ) && strpos( $_SERVER['PHP_SELF'], 'wp-admin/' ) !== false ) {
+                if ( isset( $perma_query_vars ) && strpos( $_SERVER['PHP_SELF'], qtranxf_get_admin_base() . '/' ) !== false ) {
                     unset( $perma_query_vars );
                 }
                 $wp->did_permalink = false;

src/url.php

@@ -283,14 +283,14 @@ function qtranxf_language_neutral_path( string $path ): bool {
     if ( isset( $language_neutral_path_cache[ $path ] ) ) {
         return $language_neutral_path_cache[ $path ];
     }
-    if ( preg_match( '#^/(wp-.*\.php|wp-login/|wp-admin/|xmlrpc.php|robots.txt|oauth/)#', $path ) ) {
-        $language_neutral_path_cache[ $path ] = true;
-
-        return true;
-    }
-    if ( qtranxf_ignored_file_type( $path ) ) {
+    // WordPress doesn't provide the partial path to wp-admin or login, so check those from the site URL.
+    $site_url_path = site_url( $path );
+    if ( str_starts_with( $site_url_path, admin_url() ) ||
+         $site_url_path === wp_login_url() ||
+         preg_match( '#^/(wp-.*\.php|xmlrpc.php|robots.txt|oauth/)#', $path ) ||
+         qtranxf_ignored_file_type( $path )
+    ) {
         $language_neutral_path_cache[ $path ] = true;
-
         return true;
     }
     $language_neutral_path_cache[ $path ] = false;
@@ -320,6 +320,15 @@ function qtranxf_get_url_info( string $url ): array {
     return $urlinfo;
 }
 
+/**
+ * Return the base admin url of the WordPress backend name e.g. `wp-admin` for a default install.
+ *
+ * @return string WordPress backend name
+ */
+function qtranxf_get_admin_base(): string {
+    return trim( str_replace( site_url(), '', admin_url() ), '/' );
+}
+
 /**
  * Complete urlinfo with 'path-base' according to home and site info.
  * If they differ, 'doing_front_end' might be set.