浅谈对PHP单例模式的理解以及示例代码

<span style="font-size: 14px;"><span style="font-size: 12px;"><?php</span><br/><br/><span style="font-size: 12px;">class User</span><br/><span style="font-size: 12px;">{</span><br/><span style="font-size: 12px;">    /*</span><br/><span style="font-size: 12px;">     * 1、创建一个存放对象的私有化静态变量</span><br/><span style="font-size: 12px;">     * 2、私有化克隆方法</span><br/><span style="font-size: 12px;">     * 3、私有化构造方法</span><br/><span style="font-size: 12px;">     * 4、创建实例化对象的唯一入口</span><br/><span style="font-size: 12px;">     *</span><br/><span style="font-size: 12px;">     * **/</span><br/><span style="font-size: 12px;">    private static $_instance = '';</span><br/><span style="font-size: 12px;">    private function __clone(){}</span><br/><span style="font-size: 12px;">    private function __construct(){}</span><br/><span style="font-size: 12px;">    static public function getInstance()</span><br/><span style="font-size: 12px;">    {</span><br/><span style="font-size: 12px;">        if(is_null(self::$_instance) || isset(self::$_instance)){</span><br/><span style="font-size: 12px;">            self::$_instance = new User();</span><br/><span style="font-size: 12px;">        }</span><br/><span style="font-size: 12px;">        return self::$_instance;</span><br/><span style="font-size: 12px;">    }</span><br/><span style="font-size: 12px;">    public function getIp()</span><br/><span style="font-size: 12px;">    {</span><br/><span style="font-size: 12px;">        return $_SERVER['SERVER_ADDR'];</span><br/><span style="font-size: 12px;">    }</span><br/><span style="font-size: 12px;">}</span><br/><span style="font-size: 12px;">$op =  User::getInstance();</span><br/><span style="font-size: 12px;">echo $op->getIp();</span><br/><br/></span>