Allow PHPMailer class to be reliably overriden. Refresh for 28618

src/wp-includes/pluggable.php

@@ -254,6 +254,17 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 			require_once ABSPATH . WPINC . '/class-wp-phpmailer.php';
 			$phpmailer = new WP_PHPMailer( true );
 
+			/**
+			* Filters the PHPMailer object.
+			*
+			* Allows plugins to override the PHPMailer class with one of their own if required.
+			*
+			* @since 6.9.0
+			*
+			* @param object $phpmailer A PHPMailer (or compatible) class.
+			*/
+			$phpmailer = apply_filters( 'wp_phpmailer', $phpmailer );
+
 			$phpmailer::$validator = static function ( $email ) {
 				return (bool) is_email( $email );
 			};

tests/phpunit/includes/functions.php

@@ -363,3 +363,14 @@ function _unhook_font_registration() {
 	remove_action( 'init', '_wp_register_default_font_collections' );
 }
 tests_add_filter( 'init', '_unhook_font_registration', 1000 );
+
+/**
+ * Ensure that PHPMailer is always a MockPHPMailer
+ *
+ * @param object $phpmailer The PHPMailer object.
+ * @return object The MockPHPMailer object.
+ */
+function _wp_phpmailer( $phpmailer ) {
+	return new MockPHPMailer();
+}
+tests_add_filter( 'wp_phpmailer', '_wp_phpmailer' );

tests/phpunit/tests/pluggable/wpMail.php

@@ -554,4 +554,27 @@ public function test_wp_mail_resets_properties() {
 		$phpmailer = $GLOBALS['phpmailer'];
 		$this->assertNotSame( 'user1', $phpmailer->AltBody );
 	}
+
+	/**
+	 * If the global $phpmailer is overridden, it should be recreated as a real MockPHPMailer object in tests.
+	 *
+	 * @ticket 28618
+	 */
+	public function test_global_override_can_create_real_phpmailer() {
+		global $phpmailer;
+
+		$this->assertInstanceOf( 'MockPHPMailer', $phpmailer, 'The global $phpmailer should be a MockPHPMailer.' );
+
+		$phpmailer = null;
+
+		global $phpmailer;
+
+		$this->assertNull( $phpmailer, 'The global $phpmailer should be null.' );
+
+		wp_mail( 'test@example.com', 'Test Subject', 'Test Message' );
+
+		global $phpmailer;
+
+		$this->assertInstanceOf( 'MockPHPMailer', $phpmailer, 'The global $phpmailer should still be a MockPHPMailer.' );
+	}
 }