class?User?{?
????//靜態(tài)變量保存全局實(shí)例
????private?static?$_instance?=?null;
????//私有構(gòu)造函數(shù)，防止外界實(shí)例化對(duì)象
????private?function?__construct()?{
????}????
????//靜態(tài)方法，單例統(tǒng)一訪(fǎng)問(wèn)入口
????static?public?function?getInstance()?{
????????if?(is_null?(?self::$_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?yàn)槊恳淮蝞ew操作都會(huì)消耗系統(tǒng)和內(nèi)存的資源。