<?php

class C

{

    //三私一公

    protected static $_instance = null;

    protected function __construct() //protected方便继承 ,privated无法继承

    {

        throw new Exception("禁止实例化");

    }

    protected function __clone()

    {

        throw new Exception("禁止克隆")

    }

    public function getInstance()

    {

        if (static::$_instance === null) {

            static::$_instance = new static;//后期静态绑定，以实现继承

        }

        return static::$_instance;

    }

}

class D extends C

{

    protected static $_instance = null;//继承之后能够实现两套不同的数据库链接方式

}

$c = C::getInstance();

$d = D::getInstance();

var_dump($c === $d);