include() in IE browsers?

Is there some inherent bug in IE browsers (eg. IE6) where using something like:

<?php
if (is_tree(85)) {
include('sidebar-85.php'); }
elseif (is_tree(87)) {
include('sidebar-87.php'); }
elseif (is_tree(90)) {
include('sidebar-90.php'); }
elseif (is_tree(94)) {
include('sidebar-94.php'); }
elseif (is_tree(99)) {
include('sidebar-99.php'); }
elseif (is_tree(102)) {
include('sidebar-102.php'); }
else {
get_sidebar();
}
?>

I have is_tree() in my functions and it is working elsewhere perfectly (Firefox browsers) AND is_tree() seems to be working even in IE6 for writing styles in the header conditionally.

So… is there an includes issue for IE6?