private static function _remoteIP()
    {
        // Catch all possible hints of the client's IP address, not just REMOTE_ADDR
        foreach (
            array(
                'HTTP_CLIENT_IP',
                'HTTP_X_FORWARDED_FOR',
                'HTTP_X_FORWARDED',
                'HTTP_X_CLUSTER_CLIENT_IP',
                'HTTP_FORWARDED_FOR',
                'HTTP_FORWARDED',
                'REMOTE_ADDR'
            )
            as $key
        ) {
            if (array_key_exists($key, $_SERVER)) {
                foreach (dejoin(',', $_SERVER[$key]) as $ip) {
                    if (filter_var($ip, FILTER_VALIDATE_IP)) {
                        // Return the first one found
                        return $ip;
                    };
                };
            };
        };
        return null;
    }