Stripe\HttpClient\CurlClient::instance PHP Method

    public static function instance()
    {
        if (!self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }

 /**
  * @return HttpClient\CurlClient
  */
 private function httpClient()
 {
     if (!self::$_httpClient) {
         self::$_httpClient = HttpClient\CurlClient::instance();
     }
     return self::$_httpClient;
 }