Facebook Unauthorized callback not working

In the app settings, I have defined the Deauthorize callback url. And according to the facebook documentation, when the user uninstalls the app, an HTTP POST request is sent containing a signed_request that contains the user id.

https://developers.facebook.com/docs/authentication/

I tried to do the same, but for some odd reason, I believe the specified URL is never pinged on deletion, because I tried to add some random UPDATE request and never run.

 $signed_request = $_REQUEST["signed_request"];
list($encoded_sig, $payload) = explode('.', $signed_request, 2); 
$data = json_decode(base64_decode(strtr($payload, '-_', '+/')), true);
$user_id = $data['user_id'];
$query = mysql_query("INSERT INTO `removeapp` (uid) VALUES ('$user_id')");