php 单例设计模式核心代码

<?php 
header("Content-type: text/html; charset=utf-8");
//设置中国时区
date_default_timezone_set('PRC');

class Db{
    private static $obj = null;
    private function __construct(){ }
    private function __clone(){}
    public static function getInstance(){
        if (!self::$obj instanceof self) {
            self::$obj = new self();
        }
        return self::$obj;
    }
    public function __tostring() { return "Happy birthday!";  }
}
// $db1 = new Db();
$db1 = Db::getInstance();
$db2 = Db::getInstance();
var_dump($db1,$db2);