yii2：致命错误：未捕获SoapFault异常：[8]数组到字符串转换

Question

this is my client soap api :

         $client = new SoapClient('http://sabapal.ir/sabapal/frontend/web/apit/PaymentRequest?WSDL', array('encoding' => 'UTF-8'));

        $result = $client->PaymentRequest(
                array(
                    'Amount' => 1000,
                )
        );

and this is my soap server :

    public function PaymentRequest($Amount)
{

    \Yii::$app->response->format = \yii\web\Response::FORMAT_XML;
    $xmlValues = ['test1' => 'value1', 'test2' => 'value2'];
      return $xmlValues;
}

i have this error :

Fatal error: Uncaught SoapFault exception: [8] Array to string conversion in /home/sefroweb/domains/sefroweb.com/public_html/wp-content/plugins/zarinpal-paid-downloads/zarinpalpaid-downloads.php:2890

I do not want to change client soap api

I just want to change public function PaymentRequest