Serialization error

<?php

error_reporting(E_ALL);

class Test
{
    public $v1;
    private $v2;
}

$t = new Test();
$s = serialize($t);
echo $s;

// value copied here or php won't unserialize
$s = 'O:4:"Test":2:{s:2:"v1";N;s:8:"Testv2";N;}';

$u = unserialize($s);