public static function getIpAddress()
    {
        // check for shared internet/ISP IP
        if (isset($_SERVER['HTTP_CLIENT_IP']) && self::validateIp($_SERVER['HTTP_CLIENT_IP'])) {
            return $_SERVER['HTTP_CLIENT_IP'];
        }

        // check for IPs passing through proxies
        if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
            // check if multiple ips exist in var
            if (strpos($_SERVER['HTTP_X_FORWARDED_FOR'], ',') !== false) {
                $iplist = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);
                foreach ($iplist as $ip) {
                    if (self::validateIp($ip)) {
                        return $ip;
                    }
                }
            } else {
                if (self::validateIp($_SERVER['HTTP_X_FORWARDED_FOR'])) {
                    return $_SERVER['HTTP_X_FORWARDED_FOR'];
                }
            }
        }
        if (isset($_SERVER['HTTP_X_FORWARDED']) && self::validateIp($_SERVER['HTTP_X_FORWARDED'])) {
            return $_SERVER['HTTP_X_FORWARDED'];
        }
        if (isset($_SERVER['HTTP_X_CLUSTER_CLIENT_IP']) && self::validateIp($_SERVER['HTTP_X_CLUSTER_CLIENT_IP'])) {
            return $_SERVER['HTTP_X_CLUSTER_CLIENT_IP'];
        }
        if (isset($_SERVER['HTTP_FORWARDED_FOR']) && self::validateIp($_SERVER['HTTP_FORWARDED_FOR'])) {
            return $_SERVER['HTTP_FORWARDED_FOR'];
        }
        if (isset($_SERVER['HTTP_FORWARDED']) && self::validateIp($_SERVER['HTTP_FORWARDED'])) {
            return $_SERVER['HTTP_FORWARDED'];
        }
        // return unreliable ip since all else failed
        return isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : 'unknown';
    }