Description:
------------
PDO::FETCH_SERIALIZE flag must enable automatic deserialization of an object, while it does it in some wrong way.

If you additionally see the `passed data` line you will notice that the argument passed there contains extra data (the class name) that should not be there.

It causes the whole unserialization process to be broken.

The correspondning test https://github.com/php/php-src/blob/master/ext/pdo_mysql/tests/pdo_mysql_stmt_fetch_serialize.phpt is also invalid since it does not check that we can assemble the original object back.

Test script:
---------------
class foo implements Serializable {
    private $data;
    public function __construct() {
        $this->data = "My private data";
    }
    public function serialize() {
        return serialize($this->data);
    }
    public function unserialize($data) {
        var_dump('passed data: ', $data);
        $this->data = unserialize($data);
    }
    public function getData() {
        return $this->data;
    }
}
$foo = new foo;
//var_dump(serialize($foo));

$stmt = $pdo->prepare('SELECT \'C:3:"foo":23:{s:15:"My private data";}\'');
$stmt->execute();
$stmt->setFetchMode(PDO::FETCH_CLASS|PDO::FETCH_SERIALIZE, 'foo');
$data = $stmt->fetch();
var_dump($data);

Expected result:
----------------
  object(foo)#5 (1) {
    ["data":"foo":private]=>
    string(15) "My private data"
  }

Actual result:
--------------
object(foo)#4 (1) {
  ["data":"foo":private]=>
  object(foo)#5 (1) {
    ["data":"foo":private]=>
    string(15) "My private data"
  }
}