protected function generateSignature($params, $secret) {
    // Work with sorted data.
    ksort($params);

    // Generate the base string.
    $base_string = '';
    foreach ($params as $key => $value) {
      $base_string .= $key . '=' . $value;
    }
    $base_string .= $secret;

    return md5($base_string);
  }