public static function encode($payload, $key, $alg = 'HS256', $keyId = null, $head = null)
    {
        $header = array('typ' => 'JWT', 'alg' => $alg);
        if ($keyId !== null) {
            $header['kid'] = $keyId;
        }
        if ( isset($head) && is_array($head) ) {
            $header = array_merge($head, $header);
        }
        $segments = array();
        $segments[] = JWT::urlsafeB64Encode(JWT::jsonEncode($header));
        $segments[] = JWT::urlsafeB64Encode(JWT::jsonEncode($payload));
        $signing_input = implode('.', $segments);

        $signature = JWT::sign($signing_input, $key, $alg);
        $segments[] = JWT::urlsafeB64Encode($signature);

        return implode('.', $segments);
    }