Facebook Messenger Chatbot - 我无法回复回复

I'm writing a bot for Facebook in PHP. I'm able to send text and send and answer to quickreplies. However is impossible to me to answer to a postback. I use ngrok for localhost testing purposes but I know that it's not that the problem since 1. it doesn't work on my vps neither 2. All the other functionalities work as expected.

My code for sending the post-back :

$jsonData = '{
      "recipient":{
        "id":"'.$sender.'"
      },
      "message":{
        "attachment":{
          "type":"template",
          "payload":{
            "template_type":"button",
            "text":"What do you want to do next?",
            "buttons":[
              {
                "type":"postback",
                "title":"Start Chatting",
                "payload":"USER_DEFINED_PAYLOAD"
              }
            ]
          }
        }
      }
    }';
    $url = 'https://graph.facebook.com/v2.6/me/messages?access_token='.$access_token;
$ch = curl_init($url);

//Tell cURL that we want to send a POST request.
curl_setopt($ch, CURLOPT_POST, 1);
//Attach our encoded JSON string to the POST fields.
curl_setopt($ch, CURLOPT_POSTFIELDS, $jsonData);
//Set the content type to application/json
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
//Execute the request

if(!empty($message_body['message'])){
  $result = curl_exec($ch);
}

Then, when I receive the post-back I get this from FB messenger :

{
    "object": "page",
    "entry": [
        {
            "id": "590445400970275",
            "time": 1494251031827,
            "messaging": [
                {
                    "recipient": {
                        "id": "590445400970275"
                    },
                    "timestamp": 1494251031827,
                    "sender": {
                        "id": "1075794782546272"
                    },
                    "postback": {
                        "payload": "USER_DEFINED_PAYLOAD"
                    }
                }
            ]
        }
    ]
}

and my code to parse it :

$input = json_decode(file_get_contents('php://input'), true);
$message_body = $input['entry'][0]['messaging'][0];
$sender = $message_body['sender']['id'];
if (isset($message_body['postback'])){
  //Reception d'un postback
  $postback = $message_body['postback']['payload'];
  if ($postback == "USER_DEFINED_PAYLOAD"){
    // The JSON data.
    $jsonData = '{
      "recipient":{
        "id":"'.$sender.'"
      },
      "message":{
        "text":"messagetoreply"
      }
    }';
  }
}

This is sent by the curl function as the message before.

However the messenger never receives an answer :

Could anyone help me finding out where's the problem ?

Thank you very much

There was a mistake in my code at this part :

if(!empty($message_body['message'])){
  $result = curl_exec($ch);
}

since the $message_body var is not 'message' but a 'postback', the curl was not sending the response.