理解PHP经典设计的三大模式。实例模式、工厂模式、注册树模式

<?php
class Tree
{
    public $siteName;
    protected static $instance;
    protected function __construct($siteName)
    {
        $this->siteName=$siteName;
    }
    //获取本类的唯一实例
    public static function getInstance ($siteName="dream698")
    {
        if(!self::$instance instanceof  self)
        {
            self::$instance =new self('www.feichengs.com');
        }
        return self::$instance;
    }
}
//使用工厂函数生成类
class Factory
{
    public static function create()
    {
        return Tree::getInstance();
    }
}
//使用注册树注册类
class Register
{
    public static  $objects=[];
    //往注册树上挂
    public static function set($alias,$obj)
    {
        self::$objects[$alias]=$obj;
    }
    //获取注册树的对象
    public static function get($alias)
    {
        return self::$objects[$alias];
    }
    //销毁注册时
    public static function destroy($alias)
    {
        unset($objects[$alias]);
    }
}
Register::set("site",Factory::create());
$obj=Register::get('site');
var_dump($obj);
?>