Since super-globals such as $SERVER are, well, super-global, you don't need to pass them as args to functions. So you could change it to:
function ActiveTab($page,) {
if (ereg($page, $_SERVER['REQUEST_URI'])) {
echo 'id="active_menu"';
}
}
// sample usage:
ActiveTab("sample_page_name");
If that doesn't make any difference, then I'd need to see what the values of both $page and $_SERVER['REQUEST_URI'] are.
PS: You should quote your string literals when used as array indexes (even though it will probably not matter 99% of the time -- except for throwing a "notice" type error).