php单例模式

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<?php <?php class sigle{     protected static $ins = null;     public static function getins(){         if(self::$ins === null){             self::$ins = new self();         }         return self::$ins;     } //方法千家final 则方法不能被覆盖，类前加final则类不能被继承     final protected function __construct(){       } //封锁克隆     final protected function __clone(){       } } $s1 = sigle::getins(); $s2 = clone $s1;//报错