GuzzleHttp\Client::getConfig PHP Method

    public function getConfig($option = null)
    {
        return $option === null ? $this->config : (isset($this->config[$option]) ? $this->config[$option] : null);
    }

 public function post($full_url, array $multi_parts = [], array $headers = [])
 {
     $options = ['debug' => GUZZLE_DEBUG];
     // Grab the client's handler instance.
     $clientHandler = $this->client->getConfig('handler');
     // Create a middleware that echoes parts of the request.
     $tapMiddleware = Middleware::tap(function ($request) {
         echo $request->getHeader('Content-Type');
         // application/json
         echo $request->getBody();
         // {"foo":"bar"}
     });
     //$options['handler'] = $tapMiddleware($clientHandler);
     $multi_part_vars = array();
     foreach ($multi_parts as $name => $data) {
         if (is_array($data)) {
             $data['name'] = $name;
         } else {
             $data = ['name' => $name, 'contents' => $data];
         }
         $multi_part_vars[] = $data;
     }
     $options['multipart'] = $multi_part_vars;
     //$options['headers'] = ['Referer' =>  $full_url];
     if (!empty($headers)) {
         $options['headers'] = $headers;
     }
     $this->response = $this->client->post($full_url, $options);
     return $this;
 }