Typecho 调用某一分类下的所有子分类

<?php
$obj = $this->widget('Widget_Metas_Category_List');
if($obj->have()){
  $restr = '';
  while($obj->next()){
    if($obj->parent==5){
      //do something
    }
  }
  echo $restr;
}else{
  echo '无分类';
}
?>