Duo\Web::signVals PHP Method

    private static function signVals($key, $vals, $prefix, $expire, $time = null)
    {
        $exp = ($time ? $time : time()) + $expire;
        $val = $vals . '|' . $exp;
        $b64 = base64_encode($val);
        $cookie = $prefix . '|' . $b64;
        $sig = hash_hmac("sha1", $cookie, $key);
        return $cookie . '|' . $sig;
    }