PHP DOM, class properties and passing objects by reference

Question

If objects are passed by reference in PHP5, why does the following always display 123 then xyz, as opposed to abc then xyz which is what I would expect?

<?php

class CustomDOMElement extends DOMElement
{
    public $custom_property = '123';

    public function echoCustomProperty()
    {
        var_dump($this->custom_property);
    }
}

$document = new DOMDocument();
$document->registerNodeClass('DOMElement', 'CustomDOMElement');
$document->loadHTML('<div>Hi, this is a test</div>');

$document->documentElement->custom_property = 'abc';
$document->documentElement->echoCustomProperty();

$elem = &$document->documentElement;
$elem->custom_property = 'xyz';
$elem->echoCustomProperty();

?>

Do I always have to explicitly store a reference like I have done with $elem in order to set properties on the element?