code: public function asXml($data)
      {
          $response = Yii::$app->getResponse();
          $response->format = Response::FORMAT_XML;
          $response->data = $data;
          return $response;
      }
docs: Send data formatted as XML.
  
  This method is a shortcut for sending data formatted as XML. It will return
  the [[Application::getResponse()|response]] application component after configuring
  the [[Response::$format|format]] and setting the [[Response::$data|data]] that should
  be formatted. A common usage will be:
  
  ```php
  return $this->asXml($data);
  ```
  
  @param mixed $data the data that should be formatted.
  @return Response a response that is configured to send `$data` formatted as XML.
  @since 2.0.11
  @see Response::$format
  @see Response::FORMAT_XML
  @see XmlResponseFormatter