Allow PHPMailer class to be reliably overriden. Refresh for 28618

src/wp-includes/pluggable.php

@@ -254,6 +254,17 @@
 			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' );