private function create_nonce($length=12, $include_time=true) {
    if ($this->config['force_nonce'] == false) {
      $sequence = array_merge(range(0,9), range('A','Z'), range('a','z'));
      $length = $length > count($sequence) ? count($sequence) : $length;
      shuffle($sequence);

      $prefix = $include_time ? microtime() : '';
      $this->config['nonce'] = md5(substr($prefix . implode('', $sequence), 0, $length));
    }
  }