怎么理解PHP代理模式

<?php
/*
 * 代理模式
 */

//代理抽象接口
interface shop
{
    public function buy($title);
}

//原来的CD商店，被代理对象
class CDShop implements shop
{
    public function buy($title)
    {
        echo "购买成功，这是你的《{$title}》唱片" . PHP_EOL;
    }
}

//CD代理
class Proxy implements shop
{
    public function buy($title)
    {
        $this->go();
        $CDshop = new CDshop;
        $CDshop->buy($title);
    }

        public function go()
    {
        echo "跑去香港代购" . PHP_EOL;
    }
}

class client
{
    public static function shoping($goods)
    {
        $proxy = new Proxy;
        $proxy->buy($goods);
    }
}

client::shoping("醒着做梦");