[php-src] Issue #13944: getAttribute('xmlns') returns NULL when using the new DOM API

Issue: https://github.com/php/php-src/issues/13944 Author: ausi ### Description I was rewriting some code to use the new spec compliant DOM API and noticed something odd: ->getAttribute('xmlns') seems to behave differently than the old implementation. ```php // New DOM $doc = \DOM\XMLDocument::createEmpty(); $doc->appendChild($doc->createElementNS('http://www.w3.org/2000/svg', 'svg')); var_dump($doc->firstChild->getAttribute('xmlns')); // NULL $doc2 = \DOM\XMLDocument::createFromString($doc->saveXML()); var_dump($doc2->firstChild->getAttribute('xmlns')); // string(26) "http://www.w3.org/2000/svg" // Old DOM $doc = new \DOMDocument(); $doc->appendChild($doc->createElementNS('http://www.w3.org/2000/svg', 'svg')); var_dump($doc->firstChild->getAttribute('xmlns')); // string(26) "http://www.w3.org/2000/svg" $doc2 = new \DOMDocument(); $doc2->loadXML($doc->saveXML()); var_dump($doc2->firstChild->getAttribute('xmlns')); // string(26) "http://www.w3.org/2000/svg" ``` ->getAttribute('xmlns') returns NULL at first, but after serializing and parsing the document again it returns "http://www.w3.org/2000/svg"`. I’d have expected that serializing and parsing produces an identical state of the document, but it didn’t. I was not able to find out what behavior the specification demands here for ->getAttribute('xmlns'). ### PHP Version PHP 8.4.0-dev ### Operating System macOS 14.4.1 (23E224)