扩展PDO类时对象声明出错

我创建了三个类。一个类是从PDO扩展而来的db。另外两个类是从db类扩展而来的。但问题是，当我初始化这些子类的对象时，第二个对象被创建为第一个对象的克隆。提前感谢您的帮助。

<?php
/** The Database Driver */
define('DB_DRIVER', 'mysql');
/** The name of the database */
define('DB_NAME', 'sample');
/** MySQL database username */
define('DB_USER', 'root');
/** MySQL database password */
define('DB_PASSWORD', 'root');
/** MySQL hostname */
define('DB_HOST', 'localhost');
class db extends PDO
{
    public function __construct()
    {
        echo "DB constructor called'n";
        $options = array(
            PDO::ATTR_PERSISTENT => true,
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
        );
        $dsn = DB_DRIVER . ":host=" . DB_HOST . ";dbname=" . DB_NAME;
        parent::__construct($dsn, DB_USER, DB_PASSWORD, $options);
    }
}
class Admin extends db
{
    private $uid, $username, $password, $level, $name, $email;
    public function __construct()
    {
        echo "Admin constructor called'n";
        parent::__construct();
    }
}
class Movie extends db
{
    private $mid, $title, $slug;
    public function __construct()
    {
        echo "Movie constructor called'n";
        parent::__construct();
    }
}

$adminObj = new Admin();
$movieObj = new Movie();
var_dump($adminObj);
var_dump($movieObj);
?>

这是上面的输出请注意输出中显示的对象类型。

Admin constructor called
DB constructor called
Movie constructor called
DB constructor called
object(Admin)#1 (6) {
  ["uid":"Admin":private]=>
  NULL
  ["username":"Admin":private]=>
  NULL
  ["password":"Admin":private]=>
  NULL
  ["level":"Admin":private]=>
  NULL
  ["name":"Admin":private]=>
  NULL
  ["email":"Admin":private]=>
  NULL
}
object(Admin)#2 (6) {
  ["uid":"Admin":private]=>
  NULL
  ["username":"Admin":private]=>
  NULL
  ["password":"Admin":private]=>
  NULL
  ["level":"Admin":private]=>
  NULL
  ["name":"Admin":private]=>
  NULL
  ["email":"Admin":private]=>
  NULL

}

当父级：：__construct（$dsn，DB_USER，DB_PASSWORD，$options）；删除db类后，问题将消失。