Messenger bot not working - php

Question

I am trying to set up a very simple bot. I generated an access token, created a simple index.php file on my web server (SSL certified), set up a webhook that points to my index.php file, subscribed my webhook to my page events (everything on developers.facebook.com), messaged my bot and got no answer. What might be the problem?
(I have checked everything: both tokens, I am admin etc.)
Here is my code:
<?php
$access_token = "i-filled-this-out";
$verify_token = "i-also-filled-this-out";
$hub_verify_token = null;
if(isset($_REQUEST['hub_challenge'])) {
$challenge = $_REQUEST['hub_challenge'];
$hub_verify_token = $_REQUEST['hub_verify_token'];
}
if ($hub_verify_token === $verify_token) {
echo $challenge;
}
$input = json_decode(file_get_contents('php://input'), true);
$sender = $input['entry'][0]['messaging'][0]['sender']['id'];
$message = $input['entry'][0]['messaging'][0]['message']['text'];
$message_to_reply = '';
$message_to_reply = 'Huh! what do you mean?';
//API Url
$url = 'https://graph.facebook.com/v2.6/me/messages?access_token='.$access_token;
//Initiate cURL.
$ch = curl_init($url);
//The JSON data.
$jsonData = '{
"recipient":{
"id":"'.$sender.'"
},
"message":{
"text":"'.$message_to_reply.'"
}
}';
//Encode the array into JSON.
$jsonDataEncoded = $jsonData;
//Tell cURL that we want to send a POST request.
curl_setopt($ch, CURLOPT_POST, 1);
//Attach our encoded JSON string to the POST fields.
curl_setopt($ch, CURLOPT_POSTFIELDS, $jsonDataEncoded);
//Set the content type to application/json
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
//curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/x-www-form-urlencoded'));
//Execute the request
if(!empty($input['entry'][0]['messaging'][0]['message'])){
$result = curl_exec($ch);
}

Answer 1

Make sure your webhook is returning a HTTP 200 response to the POST requests sent by facebook.
What web server are you running? Check the logs to see if your webhook is receiving POST requests from facebook (and returning a 200).
Best way of testing your webhook is using something like postman to imitate the standard POSTs made by facebook and checking the responses you are serving.