获取id的所有子级id

public function getChildrenIds ($id){
$ids = $id;
$result = D('PxCourseType','Peix')->where(array('pid'=>$id,'status'=>1))->field('id,pid,name')->select();
if ($result){
foreach ($result as $key=>$val){
$ids .= ','.$val['id'].',';
$ids .= $this->getChildrenIds ($val['id']);
}
}
return $ids;
}