Decrypts a Rails (Devise/Warden) cookie in PHP

decrypt_devise_cookie.php

function decrypt_cookie($cookie, $app_secret) {
  $token_hashed = openssl_pbkdf2($app_secret, 'encrypted cookie', 32, 1000, 'sha1');
  $encrypted_message = explode('--', Base64_decode($cookie))[0];
  $decrypted_data = openssl_decrypt($encrypted_message, 'aes-256-cbc', $token_hashed);
  return $decrypted_data;
}