[Plugin: Exec-PHP] Code not executed in a widget but ok when hard coded

I want to display subcategories of current category in a widget. So i tested exec-php and “exec-php / hello world” works fine.

The code I’m using (see below) works fine when hard-coded in the sidebar, but doesn’t display anything when i put it in a text widget.

I tried with no other active plugin, same result …

You can see the two widgets and the hard-coded result here, at the bottom of the sidebar.
http://www.xarax.fr/wpfsc4/rubrique/democratisation-de-la-science/

Any idea ? Thanks.

<?php
if (is_category()) {
  $this_category = get_category($cat);
  if (get_category_children($this_category->cat_ID) != "") {
    wp_list_categories('orderby=id&show_count=0&title_li=
&use_desc_for_title=1&child_of='.$this_category->cat_ID);
    }
}
?>