getElementById question (bug report)

Hi

I have a problem with "getElementById":

Working example:
============
-----------snip---
 <?php
$xmlstring = '<?xml version="1.0" ?>
<!DOCTYPE books [
<!ATTLIST book id ID #IMPLIED>
]>
<books>
    <book id="1">
        <title>Book 1</title>
    </book>
    <book id="2">
        <title>Book 2</title>
    </book>
</books>';

$dom = new DomDocument();
$dom->loadXML($xmlstring);
$node = $dom->getElementById("1");
print $node->nodeValue;
?>
----snap--------

NOT working example:
===============

---snip---
 <?php
$xmlstring = '<?xml version="1.0" ?>
<books>
    <book id="1">
        <title>Book 1</title>
    </book>
    <book id="2">
        <title>Book 2</title>
    </book>
</books>';

$dom = new DomDocument();
$dom->loadXML($xmlstring);
$node = $dom->getElementById("1");
print $node->nodeValue;
?>
----snap-----

===========

If there is no DTD the method "getElementById" is not working.
I don't know the DOM Standard very good, but I think this is an PHP error.

Env:
PHP5 RC2, Linux 

Bernhard