$sql = 'select * from cat order by cat_id desc';
$list = $db->getAll($sql);
$list = getLevelCat($list);
function getLevelCat($catlist, $parent_id='0', $html='???', $level='0'){
  $arr = array();
  foreach($catlist as $val){
    if($val['parent_id']==$parent_id){
      $val['html'] = str_repeat($html,$level);
      $val['level'] = $level;
      $arr[] = $val;
      $arr = array_merge($arr, getLevelCat($catlist, $val['cat_id'], $html, $level+1));
    }
  }
  return $arr;
}