<?php

class PDO2 {
    private $dsn, $username, $password;

    public function __construct($dsn, $username, $password)
    {
        $this->dsn = $dsn;
        $this->username = $username;
        $this->password = $password;
    }
}

class Connection
{
    protected $link;
    private $dsn, $username, $password;

    public function __construct($dsn, $username, $password)
    {
        $this->dsn = $dsn;
        $this->username = $username;
        $this->password = $password;
        $this->connect();
    }

    private function connect()
    {
        $this->link = new PDO2($this->dsn, $this->username, $this->password);
    }

    public function __wakeup()
    {
        $this->connect();
    }
}

$c = new Connection('aaaa', 'bbb', 'ccc');

$s = serialize($c);

print_r($s);

public function __sleep()
{
    return array('dsn', 'username', 'PDO2');
}

Notice: serialize(): "PDO2" returned as member variable from __sleep() but does not exist in

public function __sleep()
{
    return array('dsn', 'username');
}