php 链式操作的实现 学习记录

<?php
namespace commom;

class db
{
    public function where($where){
        return $this;
    }

    public function order($order){
        return $this;
    }

    public function limit($limit){
        return $this;
    }
    public function select($select){

    }
}

<?php
    $db  = new DataBase();
    $db->where()->order()->select();
?>