function CategorySelectControl(CategoryCollection $o_categories, $page_valid = null)
 {
     # set properties
     parent::XhtmlSelect('category', null, $page_valid);
     $this->SetBlankFirst(true);
     # add categories
     $a_categories = $o_categories->GetItems();
     foreach ($a_categories as $o_category) {
         $o_opt = new XhtmlOption($o_category->GetName(), $o_category->GetId());
         $o_opt->AddAttribute('style', 'padding-left: ' . ($o_category->GetHierarchyLevel() - 1) * 20 . 'px');
         $this->AddControl($o_opt);
         unset($o_opt);
     }
 }