code: private function getDownloadResponseDocument($responseType, $header)
      {
          $md5 = $this->getParsedHeader($header, 'Content-MD5');
          $requestId = $this->getParsedHeader($header, 'x-amz-request-id');
  
          $response = '<' . $responseType . 'Response xmlns="http://mws.amazonaws.com/doc/2009-01-01/">';
  
          $response .= '<' . $responseType . 'Result>';
          $response .= '<ContentMd5>';
          $response .= $md5;
          $response .= '</ContentMd5>';
          $response .= '</' . $responseType . 'Result>';
          $response .= '<ResponseMetadata>';
          $response .= '<RequestId>';
          $response .= $requestId;
          $response .= '</RequestId>';
          $response .= '</ResponseMetadata>';
          $response .= '</' . $responseType . 'Response>';
  
          return $response;
      }
docs: For MarketplaceWebService_RequestType::POST_DOWNLOAD actions, construct a response containing the Amazon Request ID
  and Content MD5 header value.
  
  @param string $responseType
  @param string $header
  @return string