code: private function responseCallback($ch, $string) {
          $httpStatusCode = (int) curl_getinfo($this->curlClient, CURLINFO_HTTP_CODE);
  
          // For unsuccessful responses, i.e. non-200 HTTP responses, we write the response body
          // into a separate stream.
          if ($httpStatusCode == 200) {
              $responseHandle = $this->responseBodyContents;
          } else {
              $responseHandle = $this->errorResponseBody;
          }
  
          return fwrite($responseHandle, $string);
      }
docs: cURL callback to write the response HTTP body into a stream. This is only intended to be used
  with MarketplaceWebService_RequestType::POST_DOWNLOAD request types, since the responses can potentially become
  large.
  
  @param $ch - The curl handle.
  @param $string - body portion to write.
  @return int - number of byes written.