Here are some links to other posts that show how to pass a Smarty variable to a PHP page. Hopefully one will do the trick for you:
http://forum.x-cart.com/showthread.php?t=13768
and
http://forum.x-cart.com/showthread.php?t=738
I also successfully passed a variable between PHP and Smarty Template pages via the instructions on my Post #60 in this thread:
http://forum.x-cart.com/showthread.php?t=6888&page=6
In that example, I'm appending the variable onto the end of the URL to get it from one page to another, so it's not safe for any sensitive information, but good for non-descript data.