PHP實現單例模式

<?php
/**
* 單例模式實現
*/
class Singleton
{
    //靜態變量保存全局實例
    private static $instance = null;

    private function __clone()
    {
        //私有構造函數，防止外界實例化對象
    }

    private function __construct()
    {
        //私有克隆函數，防止外界克隆對象
    }

    //靜態方法，單例統一訪問入口
    public static function getInstance()
    {
        if (self::$instance instanceof Singleton) {
            echo "return exist instance\n";
            return self::$instance;
        }
        self::$instance = new Singleton();
        echo "return new instance\n";
        return self::$instance;
    }
}

$a = Singleton::getInstance();//output: return new instance
$a = Singleton::getInstance();//output: return exist instance