Hmm... I don't understand what you are trying to do. Are you trying to pass a variable to the PHP w/o using forms and without it appearing in the URL/address bar?
A variable can only be sent "hidden" from the user's perpective if it is POSTed. You could always use frames so that the user wouldn't see the variable in the address bar. Use a top frame or height 1 and the bottom frame can be the main content frame.
-sridhar