Use Wordpress maintenance mode instead of custom one

template/web/wp-content/mu-plugins/studiometa-maintenance-mode/README.md

@@ -1,6 +1,6 @@
 # Studio Meta maintenance mode
 
-Put your site under maintenance with a 503 HTTP status and a dedicated page.
+Put your site under maintenance depends on server environment variables.
 
 ## Usage
 
@@ -9,6 +9,6 @@ Put your site under maintenance with a 503 HTTP status and a dedicated page.
 MAINTENANCE_ENABLED=true|false # Enable/Disable maitenance mode.
 MAINTENANCE_IPS=42.42.42.42.42 # Exclude a list of IPs from maitenance.
 ```
-2. Customize the maintenance page thanks to `maintenance.php` and `maintenance.css` files.
+2. You can customize the maintenance page by adding a `maintenance.php` file in `WP_CONTENT_DIR` (see https://developer.wordpress.org/reference/functions/wp_maintenance/).
 
 > If you use a cache plugin, don't forget to clean caches.

template/web/wp-content/mu-plugins/studiometa-maintenance-mode/class-studiometamaintenancemode.php

@@ -18,7 +18,7 @@ class StudiometaMaintenanceMode {
 	 * Constructor.
 	 */
 	public function __construct() {
-		add_action( 'init', array( $this, 'redirect_to_maintenance' ) );
+		add_filter( 'enable_maintenance_mode', array( $this, 'is_maintenance_mode' ) );
 	}
 
 	/**
@@ -31,7 +31,6 @@ protected function is_maintenance_mode() {
 			'true' !== getenv( 'MAINTENANCE_ENABLED' )
 			|| is_admin()
 			|| current_user_can( 'administrator' )
-			|| ! file_exists( __DIR__ . '/maintenance.php' )
 			|| 'wp-login.php' === $GLOBALS['pagenow']
 		) {
 			return false;
@@ -46,24 +45,6 @@ protected function is_maintenance_mode() {
 
 		return true;
 	}
-
-	/**
-	 * Redirect to maintenance mode.
-	 *
-	 * @return void
-	 */
-	public function redirect_to_maintenance() {
-		if ( ! $this->is_maintenance_mode() ) {
-			return;
-		}
-
-		$protocol = isset( $_SERVER['SERVER_PROTOCOL'] ) && 'HTTP/1.1' === $_SERVER['SERVER_PROTOCOL'] ? 'HTTP/1.1' : 'HTTP/1.0';
-
-		header( $protocol . ' 503 Service Unavailable', true, 503 );
-		header( 'Retry-After: 3600' );
-		include_once __DIR__ . '/maintenance.php';
-		die();
-	}
 }
 
 new StudiometaMaintenanceMode();