[php-src] Issue #11833: Importing DOM node doesn't set correct default namespace

Issue: https://github.com/php/php-src/issues/11833 Author: RemkoNolten ### Description In PHP 8.1.21, the behavior of importing DOM nodes from other DOMDocument instances changed. I'm not 100% sure it's a bug, but it did break backwards compatibility in our case and maybe other people are running into similar issues, so I wanted to report it nonetheless. To explain it briefly: We have a MathML document which contains a SVG as a fallback. The SVG has a svg: prefix/namespace to distinguish the MathML part from the SVG part. When we imported the SVG node to a new DOMDocument, it (correctly?) removed the svg: prefix in the resulting XML, so we could render it in the browser. Since version 8.1.21, the svg: prefix isn't removed anymore, so the browser doesn't recognize it and fails to render it. I was not able to find a workaround using createDocumentFragment or setting/removing the xmlns attribute(s) so far... The following code: ```php <?php $xml = <<<XML <?xml version="1.0"?> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" xmlns:svg="http://www.w3.org/2000/svg"> <m:annotation-xml encoding="image/svg+xml"> <svg:svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1"> </svg:svg> </m:annotation-xml> </m:math> XML; // Create a DOM Document with the above MathML+SVG: $dom = new DOMDocument(); $dom->loadXml($xml); $svg = $dom->getElementsByTagName('svg')->item(0); // Create a new DOM Document and import the SVG node $dom2 = new DOMDocument(); $importedNode = $dom2->importNode($svg); $dom2->appendChild($importedNode); // Output the results echo $dom2->saveXml(); ``` Resulted in this output: ``` <?xml version="1.0"?> <svg:svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:svg="http://www.w3.org/2000/svg" version="1.1"/> ``` But I expected this output instead: ``` <?xml version="1.0"?> <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:svg="http://www.w3.org/2000/svg" version="1.1"/> ``` Or even better: ``` <?xml version="1.0"?> <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1"/> ``` I've created a Online PHP example: https://onlinephp.io/c/90a9b - 3v4l.org doesn't support 8.1.21 (yet). ### PHP Version PHP 8.1.21 ### Operating System Debian Bullseye