PHP实现单例模式

<?php
/**
* 单例模式实现
*/
class singleton
{
    //静态变量保存全局实例
    private static $instance = null;

    private function __clone()
    {
        //私有构造函数，防止外界实例化对象
    }

    private function __construct()
    {
        //私有克隆函数，防止外界克隆对象
    }

    //静态方法，单例统一访问入口
    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