public static function wxj_category($list, $cat_id = 0, $format = "<option %s>%s</option>")
 {
     $select = "";
     foreach ($list as $key => $value) {
         $empty = "";
         for ($i = 1; $i < $value[level]; $i++) {
             $empty .= '&nbsp;&nbsp;';
         }
         $select .= sprintf($format, "value='" . $value['id'] . "' " . ($cat_id == $value['id'] ? 'selected' : ''), $empty . '├─' . $value['catname']);
         if ($value['_child']) {
             $select .= CategoryModel::wxj_category($value['_child'], $cat_id);
         }
     }
     return $select;
 }