Unique User Hashes

Hi, I am busy building an application that requires one time voting and to get around the user deleting a cookie that I set im keeping a hash on my side which I then try match before allowing anything. This is how I currently generate my hash: /* Get vars */ $browser = $_SERVER['HTTP_USER_AGENT']; $ip = $_SERVER['REMOTE_ADDR']; $real_client_ip = $_SERVER['HTTP_X_FORWARDED_FOR']; if(!empty($real_client_ip)) $name = gethostbyaddr($real_client_ip); else $name = gethostbyaddr($ip); /* Return generated hash */ return md5($browser.$ip.$real_client_ip.$name); Now thats not ideal because you can just change your user agent and vote again - but IP isnt good enough either because in South Africa due to the poor internet we all connect to international sites via a series of Transparent proxies which makes everyone seem to come from one IP. Anyone had to deal with this in the past and does anyone have any suggestions/ideas as to how I could better this setup? Many thanks in advance, Ian