php recursive fucntion to get categories and subcategories

Here is my simple recursive function to create a category subcategory tree of unlimited subcategories.

<select name="parent_cat">
       <option value="0">Root</option>
       <?
          function get_all_sub_cats($parent_cat_id, $level_string)
          {
              $return_str='';
              if(!$level_string)
              {
                  $level_string='';
              }
              $db=new DB();
              $db->open();
              $db->query("select * from categories where parent_id='{$parent_cat_id}'");
              $db->rsset();
              if($db->rs)
              {
                  while($db->rs)
                  {
                      $return_str.="<option value=\"{$db->rs['id']}\" style=\"padding-left:10px\">{$level_string}{$db->rs['name']}</option>";
                      $return_str.=get_all_sub_cats($db->rs['id'], $level_string.'+');
                      $db->rsset();
                  }
              }
              else
              {
                  return false;
              }
              return $return_str;
          }
           print get_all_sub_cats('0', '');
       ?>
      </select>

This is to fit my needs. You can get the logic and build your own accordingly.