对象序列化 - 问题

请看下面给出的代码。

class Connection
{
    protected $link;
    private $server, $username, $password, $db, $cnt;
    public function __construct($server, $username, $password, $db)
    {
        $this->server = $server;
        $this->username = $username;
        $this->password = $password;
        $this->db = $db;
        $this->cnt = 1;
        $this->connect();
    }
    public function connect()
    {
        echo '<br /> in connect '.($this->cnt++);
        $this->link = mysql_connect($this->server, $this->username, $this->password);
        mysql_select_db($this->db, $this->link);
    }
    public function __sleep()
    {
        echo '<br />in sleep';
        return array('server', 'username', 'password', 'db');
    }
    public function __wakeup()
    {
        echo '<br /> in wake up';
        $this->connect();
    }
}
$obj = new Connection('server', 'test', 'test', 'test');
$s = serialize($obj);
$obj->connect();
unserialize($s);

如果我没记错的话，序列化应该摧毁类的所有其他成员。

 $s = serialize($obj);
    $obj->connect();
    unserialize($s);

序列化"服务器"，"用户名"，"密码"，"数据库"属性后，"CNT"应销毁。 但是当我调用$obj->连接（）时; 它给了我 $this->cnt 值....

请解释一下