gistfile1.php

<?php

define('VERIFY_TOKEN', 'pancakes');
$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') {
    $updates = json_decode(file_get_contents("php://input"), true);
    file_put_contents('updates.txt',print_r($updates, true), FILE_APPEND);
}

?>