My guess would be you didn't preregister the variable for use in the footer template. Cellarius wrote a really good article that you may be interested in -
[vB4] Rendering templates and registering variables - a short guide Also, I'm not even sure if that will do the trick since the footer gets rendered pretty early on (when global.php is called). So, your best bet is to use a template_hook (may have to add one) instead.