php设计模式Singleton(单例模式)

<?php 
/** 
* 单例模式 
* 
* 保证一个类仅有一个实例,并提供一个访问它的全局访问点 
* 
*/ 
class Singleton 
{ 
static private $_instance = null; 

private function __construct() 
{ 
} 

static public function getInstance() 
{ 
if(is_null(self::$_instance)) { 
self::$_instance = new Singleton(); 
} 
return self::$_instance; 
} 

public function display() 
{ 
echo "it is a singlton class function"; 
} 
} 

// $obj = new Singleton(); // 声明不能成功 
$obj = Singleton::getInstance(); 
var_dump($obj); 
$obj->display(); 

$obj1 = Singleton::getInstance(); 
var_dump(($obj === $obj1));