you could add the variable to a session like so:
//VERY FIRST Line of page
session_start();
$_SESSION["foo"] = "bar";
echo $_SESSION["foo"] # Prints bar
Two notes:
make sure that session_start(); is the very first thing on your document before any html or even blank spaces.
This stores the data in a session which is then available to any page until the session expires or the browser is closed.
If you so wished you could also use cookies, but they IMHO are more difficult.