class?User?{?
????//靜態變量保存全局實例
????private?static?$_instance?=?null;
????//私有構造函數，防止外界實例化對象
????private?function?__construct()?{
????}????
????//靜態方法，單例統一訪問入口
????static?public?function?getInstance()?{
????????if?(is_null?(?self::$_instance?)?||?isset?(?self::$_instance?))?{
????????????self::$_instance?=?new?self?();
????????}
????????return?self::$_instance;
????}
}

使用單例模式可以避免大量的new操作。因為每一次new操作都會消耗系統和內存的資源。