add admin notice when invalid COOKIE_DOMAIN is set

inc/admin/class-configuration-checker.php

@@ -0,0 +1,72 @@
+<?php
+/**
+ * Configuration Checker for WordPress Multisite setup issues.
+ *
+ * @package WP_Ultimo
+ * @subpackage Admin
+ * @since 2.4.7
+ */
+
+namespace WP_Ultimo\Admin;
+
+// Exit if accessed directly
+defined('ABSPATH') || exit;
+
+/**
+ * Checks for common configuration issues that can affect multisite installations.
+ *
+ * @since 2.4.7
+ */
+class Configuration_Checker {
+
+	use \WP_Ultimo\Traits\Singleton;
+
+	/**
+	 * Initialize the class and register hooks.
+	 *
+	 * @since 2.4.7
+	 * @return void
+	 */
+	public function init(): void {
+
+		add_action('admin_init', [$this, 'check_cookie_domain_configuration']);
+	}
+
+	/**
+	 * Checks for COOKIE_DOMAIN configuration issues on subdomain multisite installs.
+	 *
+	 * When COOKIE_DOMAIN is defined as false on a subdomain multisite installation,
+	 * it can cause authentication and session issues across subdomains.
+	 *
+	 * @since 2.4.7
+	 * @return void
+	 */
+	public function check_cookie_domain_configuration(): void {
+		if (! is_network_admin()) {
+			return;
+		}
+		// Only check on subdomain installs
+		if ( ! is_subdomain_install()) {
+			return;
+		}
+
+		// Check if COOKIE_DOMAIN is defined and set to false
+		if (defined('COOKIE_DOMAIN') && COOKIE_DOMAIN === false) {
+			$message = sprintf(
+				// translators: %1$s is 'wp-config.php', %2$s is 'COOKIE_DOMAIN', %3$s is 'false'
+				esc_html__('Your %1$s has %2$s set to %3$s, which can cause authentication and session issues on subdomain multisite installations. Please remove this line from your wp-config.php file or set it to an appropriate value.', 'ultimate-multisite'),
+				'<strong>wp-config.php</strong>',
+				'<strong>COOKIE_DOMAIN</strong>',
+				'<strong>false</strong>',
+			);
+			$message .= '<br><a href="https://developer.wordpress.org/apis/wp-config-php/#set-cookie-domain" target="_blank" rel="noopener noreferrer">' . esc_html__('Learn more about cookie settings', 'ultimate-multisite') . ' &rarr;</a>';
+
+			\WP_Ultimo()->notices->add(
+				$message,
+				'warning',
+				'network-admin',
+				'cookie_domain_false_warning'
+			);
+		}
+	}
+}

inc/class-wp-ultimo.php

@@ -423,6 +423,11 @@ public function load_public_apis(): void {
 		 */
 		if (is_admin()) {
 			require_once wu_path('inc/functions/admin.php');
+
+			/*
+			 * Configuration Checker for multisite setup issues
+			 */
+			\WP_Ultimo\Admin\Configuration_Checker::get_instance();
 		}
 	}