PHP example for generating challenge response for Twitter webhooks

challenge-response.php

<?php
// Example app consumer secret found in apps.twitter.com
const APP_CONSUMER_SECRET = 'z3ZX4v7mAAUGykl3EcmkqbartmuW8VFOOzCloLx9Q45P0hLrFu';
// Example token provided by incoming GET request
$token = $_GET['crc_token'];
/**
 * Creates a HMAC SHA-256 hash created from the app TOKEN and
 * your app Consumer Secret.
 * @param  token  the token provided by the incoming GET request
 * @return string
 */
function get_challenge_response($token) {
  $hash = hash_hmac('sha256', $token, APP_CONSUMER_SECRET, true);
  $response = array(
    'response_token' => 'sha256=' . base64_encode($hash)
  );
  return json_encode($response);
}
// prints result
echo get_challenge_response($token);