yourphp超出20记录自动删除

$m = M('service_loginlog');
        $res =$m->where('card_id='.$_SESSION['card_id'])->order('time desc')->select();
        foreach($res as $k=>$v){
            if($k>20){
                $m->where('id='.$v['id'])->delete();        
            }
        }

用foreach循环，当$k>20的是后，就执行删除命令