If I understand you correctly.....
it depends on how the script is accessing the get vars $foo and $bar. Without looking at the script there is not really anyway to know.
i.e. they may assume that register globals is not on and so access all their variables using the approriate HTTP_POST_VARS or HTTP_GET_VARS arrays, or maybe directly as you presume, or maybe through the $GLOBALS array (although this should work as well as long as your variables are declared within the global scope).
Without knowing more about bar.php it would be impossible to say