php遞歸獲取頂級父類id

function get_top_parentid($id){
$r = M('navclass')->where('id = '.$id)->field('id,f_id')->find();
if($r['f_id'] != '0') return get_top_parentid($r['f_id']);
return $r['id'];
}

echo get_top_parentid($postcid);