code: private function sendRequest($request, $endpoint, $operation, $wsdl)
      {
          $endpointurl = $this->compileEndpointUrl($endpoint);
          $this->response = $this->getRequest()->request(
              $this->createAccess(), $request, $endpointurl, $operation, $wsdl
          );
          $response = $this->response->getResponse();
  
          if (null === $response) {
              throw new Exception('Failure (0): Unknown error', 0);
          }
  
          return $this->formatResponse($response);
      }
docs: Creates and sends a request for the given data. Most errors are handled in SoapRequest
  
  @param string $request
  @param string $endpoint
  @param string $operation
  @param string $wsdl
  
  @throws Exception
  
  @return \stdClass