code: public function register(Container $app)
      {
          $app['guzzle.handler_stack'] = function () {
              $stack = HandlerStack::create();
  
              return $stack;
          };
  
          $app['guzzle'] = function () use ($app) {
              $client = new HttpClient([
                  'handler' => $app['guzzle.handler_stack']
              ]);
  
              return $client;
          };
      }
docs: Register Guzzle with Silex
  
  @param Container $app App container to register with