Description:
------------
Setting custom variables in DOM objects like:

$domNode->customVar = 'myValue';

Are inconsistently lost when working with a dom.

Reproduce code:
---------------
<?php

class myclass {

	function run() {
		$xp = new domxpath($this->dom);

		$widths = $xp->query('//td/@width');
		foreach ($widths as $width) {
			echo $this->name.' 1: width->myVar value:'.$width->myVar.'<br>';
			$width->myVar = '1';
		}		
		$tds = $this->dom->getElementsByTagName('td');
		foreach ($tds as $td) {
			echo $this->name.' 2: td->myVar value:'.$td->myVar.'<br>';
			$td->myVar = '1';
		}
		
		echo '<br>';
		$tds = $this->dom->getElementsByTagName('td');
		foreach ($tds as $td) {
			if ($width = $td->getAttributeNode('width')) {
				echo $this->name.' 3: width->myVar value:'.$width->myVar.'<br>';
				$width->myVar = '2';
			}
			echo $this->name.' 4: td->myVar value:'.$td->myVar.'<br>';
			$td->myVar = '2';
		}
		echo '<br>';
	}
}

$xml = '<root>
		   <tr><td width="40">hello</td><td width="40">world</td></tr>
		   <tr><td>Just one TD</td></tr>
		   <tr><td>TD with <b>mixed</b> <i>content</i></td></tr>
		</root>';

$dom = new DomDocument;
$dom->loadXML($xml);

$firstInstance = new myclass;
$firstInstance->name = 'firstInstance';
$firstInstance->dom = $dom;
$firstInstance->run();
$secondInstance = new myclass;
$secondInstance->name = 'secondInstance';
$secondInstance->dom = $firstInstance->dom;
$secondInstance->run();

?>

Expected result:
----------------
firstInstance 1: width->myVar value:
firstInstance 1: width->myVar value:
firstInstance 2: td->myVar value:
firstInstance 2: td->myVar value:
firstInstance 2: td->myVar value:
firstInstance 2: td->myVar value:

firstInstance 3: width->myVar value:1
firstInstance 4: td->myVar value:1
firstInstance 3: width->myVar value:1
firstInstance 4: td->myVar value:1
firstInstance 4: td->myVar value:1
firstInstance 4: td->myVar value:1

secondInstance 1: width->myVar value:2
secondInstance 1: width->myVar value:2
secondInstance 2: td->myVar value:2
secondInstance 2: td->myVar value:2
secondInstance 2: td->myVar value:2
secondInstance 2: td->myVar value:2

secondInstance 3: width->myVar value:1
secondInstance 4: td->myVar value:1
secondInstance 3: width->myVar value:1
secondInstance 4: td->myVar value:1
secondInstance 4: td->myVar value:1
secondInstance 4: td->myVar value:1

Actual result:
--------------
firstInstance 1: width->myVar value:
firstInstance 1: width->myVar value:
firstInstance 2: td->myVar value:
firstInstance 2: td->myVar value:
firstInstance 2: td->myVar value:
firstInstance 2: td->myVar value:

firstInstance 3: width->myVar value:1
firstInstance 4: td->myVar value:
firstInstance 3: width->myVar value:1
firstInstance 4: td->myVar value:
firstInstance 4: td->myVar value:
firstInstance 4: td->myVar value:

secondInstance 1: width->myVar value:
secondInstance 1: width->myVar value:
secondInstance 2: td->myVar value:
secondInstance 2: td->myVar value:
secondInstance 2: td->myVar value:
secondInstance 2: td->myVar value:

secondInstance 3: width->myVar value:1
secondInstance 4: td->myVar value:
secondInstance 3: width->myVar value:1
secondInstance 4: td->myVar value:
secondInstance 4: td->myVar value:
secondInstance 4: td->myVar value: