facebook_subscribe.php

<?php

define('VERIFY_TOKEN', 'yourappssecret');
$method = $_SERVER['REQUEST_METHOD'];

if ($method == 'GET' && $_GET['hub_mode'] == 'subscribe' && $_GET['hub_verify_token'] == VERIFY_TOKEN) {
    echo $_GET['hub_challenge'];
} else if ($method == 'POST') {
    $updates = json_decode(file_get_contents("php://input"), true);

    // Here you can do whatever you want with the JSON object that you receive from FaceBook.
    // Before you decide what to do with the notification object you might aswell just check if
    // you are actually getting one. You can do this by choosing to output the object to a textfile.
    // It can be done by simply adding the following line:
    // file_put_contents('/filepath/updates.txt',$updates, FILE_APPEND);

    error_log('updates = ' . print_r($obj, true));
}

?>