$IP = $HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"];
$proxy = $HTTP_SERVER_VARS["REMOTE_ADDR"];
$host = @gethostbyaddr($HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"]);
You'll get something that will look like this...
313.88.1.99 <--IP addy of user.
62.31.224.1 <--IP addy of server.
313-88-1-99.bas.cvx.blueyonder.co.uk <-- Users host addy.
The host address will look slightly different depending on the ISP/server.
eg.
cache01.drkw.com
cache-mtc-ak02.proxy.aol.com
furness-gw.phila.k12.pa.us
194.238.179.180