code: public function setHttpClient($httpClient) {
      if (is_null($httpClient)) {
        $httpClient = new Curl;
        $threeScaleVersion = new ThreeScaleVersion();
  
        $version = $threeScaleVersion->getVersion();
  
        $httpClient->options['CURLOPT_FOLLOWLOCATION'] = false;
        $httpClient->headers['X-3scale-User-Agent'] = 'plugin-php-v'. $version;
      }
  
      $this->httpClient = $httpClient;
    }
docs: Curl library, which is just a thin wrapper around php's curl functions.