php单例模式

<?php
class single{
    static private $instance; //创建静态的私有变量保存该类对象
    //防止直接创建对象
    private function __construct()
    {
        echo "我被实例化了";
    }
    //防止克隆对象
    private function __clone()
    {

    }
    static public function getInstance()
    {
        //判断是否已经创建对象
        if(!self::$instance instanceof self)
        {
            self::$instance = new self();
        }
        return self::$instance;
    }
}
$db1 = single::getInstance();
$db2 = single::getInstance();