Order.php

<?php

class Order
{
    public $user1;
    
    public $user2;
}

test.php

<?php

$user = new User();
$user->username = 'foo';

$oder = new Order();
$oder->user1 = $user;
$oder->user2 = $user;

var_dump($order->user1->username, $oder->user2->username); // oba zwrócą foo

$order->user1->username = 'bar';

var_dump($order->user1->username, $oder->user2->username); // oba zwrócą bar, bo przecież user1 i user2 wskazują na ten sam obiekt

User.php

<?php

class User
{
    public $username;
}