php-object-reference.php

<?php
$foo = new StdClass();

$foo->bar = new StdClass();

$foo->bar->baz = 'toto';

// this contains the main object
var_dump($foo);

$bar = $foo->bar;

$bar->bak = 'bak';

// the sub object has been updated as expected
var_dump($bar);

// the main object has been updated too because $bar contains a reference to $foo->bar
var_dump($foo);