$HTTP_SESSION_VARS is not an auto-global, if I remember correctly. That means it is outside the scope of any function or method in your script. You could use register_globals, but that assumes your sysadmin will allow you to set PHP directives. If you can't do that, try putting this line in every function or method that refers to the session variables:
Code:
function my_func() {
global $HTTP_SESSION_VARS;
echo $HTTP_SESSION_VARS['some_var'];
}