Get client's real IP

get_client_ip

function get_client_ip() {
    $check_order  = array('HTTP_CLIENT_IP', 'HTTP_X_FORWARDED_FOR', 'HTTP_X_FORWARDED', 'HTTP_FORWARDED_FOR', 'HTTP_FORWARDED', 'REMOTE_ADDR');
    $reserved_ips = array(
        array('0.0.0.0','2.255.255.255'),
        array('10.0.0.0','10.255.255.255'),
        array('127.0.0.0','127.255.255.255'),
        array('169.254.0.0','169.254.255.255'),
        array('172.16.0.0','172.31.255.255'),
        array('192.0.2.0','192.0.2.255'),
        array('192.168.0.0','192.168.255.255'),
        array('255.255.255.0','255.255.255.255')
    );

    for($i=0; $i<count($check_order); $i++) {
        $item = $check_order[$i];
        if (isset($_SERVER[$item])) {
            $lineitem = explode(',', $_SERVER[$item]);
            $ip = trim($lineitem[0]);

            // check its not an internal ip
            if (!empty($ip) && ip2long($ip) != -1) {
                foreach ($reserved_ips as $r) {
                    $min = ip2long($r[0]);
                    $max = ip2long($r[1]);
                    if ((ip2long($ip) >= $min) && (ip2long($ip) <= $max)) {
                        return null;
                    }
                }
                return $ip;
            }
        }
    }
    return null;
}

I'm confused why you wouldn't get this directly through one of PHP's session variables?