Description:
------------
a domxml element has "0" field where its unique id lays. When you call a method, those ids change, below is the code to reproduce this.

I used this XML string:
<?xml version="1.0"?>
<test>
	<row>
		<col>a</col>
		<col>b</col>
		<col>b2</col>
	</row>
	<col>c</col>
	<col>d</col>
</test>

Reproduce code:
---------------
$doc = domxml_open_file('test.xml');

//$element = $doc->first_child();

$xp = $doc->xpath_new_context();
$res = $xp->xpath_eval('//col', $xp);

print_r($res->nodeset);

Expected result:
----------------
comment/uncomment the 3rd line you'll see that ids change