分析以下代码，判断下面说法中哪个是正确的（ ）。class MySQLDB { private static $instance;//保存实例化好的对象 private $conn; private function __construct() { $this->conn=mysql_connect('127.0.0.1','root',''); } public static function getInstance() { //判断是否$instance 属性内是否已经保存了该类的实例 if (!isset(self::$instance)) { self::$instance = new self; } return self::$instance;//返回对象 } private function __clone() { }}$db1 = MySQLDB::getInstance();$db2 = MySQLDB::getInstance();var_dump($db1, $db2);