Identifying clients?

Having experimented with REMOTE_ADDR, HTTP_X_FORWARDER_FOR, HTTP_CLIENT_IP etc. I'm still unsure as to a foolproof method of identifying clients WITHOUT having them log in to a site. I can only assume that some combination of the above environment variables and judicious use of small cookies is as near as I can get to perfection, but I'm stumped if I can see how to do it. I am aware that whatever routine will only (roughly) identify a machine and not an actual human (can users be called human?), but that will have to do. As for sample code, I can only presume it would be something like: // Step 1 - get all interesting IP addresses... $ip1 = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : null; $ip2 = isset($_SERVER['HPPT_CLIENT_IP']) ? $_SERVER['HPPT_CLIENT_IP'] : null; $ip3 = isset($_SERVER['HTTP_X_FORWARDED_FOR']) ? $_SERVER['HTTP_X_FORWARDED_FOR'] : null; // This may in fact be good enough... $ip_md5 = md5(serialize(array($ip1, $ip2, $ip3))); // Step 2 - get a cookie $cookie = isset($_COOKIE[$ip_md5]) ? $_COOKIE[$ip_md5] : null; if (is_null($cookie)) { // set the cookie with something - probably just the $ip_md5 value? } Any suggestions etc. most appreciated.