public function actionNotify(string $transactionId = null)
    {
        $transaction = $this->checkNotify($transactionId);
        if ($transaction === null) {
            return 'Unknown transaction';
        }

        Yii::$app->response->format = Response::FORMAT_RAW;

        return $transaction->isConfirmed() ? 'OK' : $transaction->getParameter('error');
    }