[ADDiCT]

$_SERVER["REMOTE_ADDR"] works fine, have used it already in "ioHTTPD" on LAN-parties. On the internet u might have to deal with proxyservers (force users to disable them for the RSS-feed).

Edit: something i found between al my crappy code:

    function FindUserIdByIP($ip)
    {
        $resultid = -1; // default return value
        
        $userlist = io_user_list_init();             // create userlist
        $usermask = io_user_mask_init(UINFO_IPLIST); // create usermask; we only need list of IP's
        
        // fetch userid's
        $uid = io_user_list_fetch($userlist);
        while($resultid == -1 && $uid >= 0)
        {
            $userfile = io_user_open($uid);            // open userfile
            $a = io_user_query($userfile, $usermask);  // get info
            io_user_close($userfile);                  // close userfile

            // check all IP's
            for ($i=1; $i <= $a[0]; $i++)
            {
                // extract ip from "ident@ip" format
                if (strpos($a[$i], "@") > 0) { $userip = substr($a[$i], strpos($a[$i],"@")+1, strlen($a[$i])); }
                                        else { $userip = $a[$i]; }
                if ($userip == $ip)
                {
                    $resultid = $uid;
                    break;
                }
            }
            
            // get next userid from list
            $uid = io_user_list_fetch($userlist);
        }
        
        io_user_mask_close($usermask); // close usermask
        io_user_list_close($userlist); // close userlist
        
        return $resultid;
    }