Symfony\Component\Serializer\Serializer::encode PHP Method

    public final function encode($data, $format)
    {
        return $this->getEncoder($format)->encode($data, $format);
    }

 /**
  * Executes a login HTTP request.
  *
  * @param string $name
  *   The username.
  * @param string $pass
  *   The user password.
  * @param string $format
  *   The format to use to make the request.
  *
  * @return \Psr\Http\Message\ResponseInterface The HTTP response.
  *   The HTTP response.
  */
 protected function loginRequest($name, $pass, $format = 'json')
 {
     $user_login_url = Url::fromRoute('user.login.http')->setRouteParameter('_format', $format)->setAbsolute();
     $request_body = [];
     if (isset($name)) {
         $request_body['name'] = $name;
     }
     if (isset($pass)) {
         $request_body['pass'] = $pass;
     }
     $result = \Drupal::httpClient()->post($user_login_url->toString(), ['body' => $this->serializer->encode($request_body, $format), 'headers' => ['Accept' => "application/{$format}"], 'http_errors' => FALSE, 'cookies' => $this->cookies]);
     return $result;
 }