function token($prefix = "")
{
    $str = md5(uniqid(mt_rand(), true));
    $_id = substr($str, 0, 8) . '-';
    $_id .= substr($str, 8, 4) . '-';
    $_id .= substr($str, 12, 4) . '-';
    $_id .= substr($str, 16, 4) . '-';
    $_id .= substr($str, 20, 12);
    return $prefix . $_id;
}