<?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);