[8.4+] Misbehavior between old DOMDocument and new Dom\HTMLDocument in XML handling

[stloyd]

Description

The following code:

<?php

$xml = <<<'XML'
<Invoice xmlns:cec="urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2"
         xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"
         xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
         xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"
         xmlns:sh="http://www.unece.org/cefact/nodes/StandardBusinessDocumentHeader">
</Invoice>
XML;

$newDocument = \Dom\HTMLDocument::createFromString($xml, LIBXML_HTML_NOIMPLIED | LIBXML_NOERROR);
$oldDocument = new \DOMDocument();
$oldDocument->loadXML($xml);

var_dump($newDocument->documentElement->namespaceURI, $oldDocument->documentElement->namespaceURI);

Resulted in this output:

string(28) "http://www.w3.org/1999/xhtml"
string(54) "urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"

But I expected this output instead:

string(54) "urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"
string(54) "urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"

---

Side note: Using \Dom\XmlDocument returns correct namespace.

---

PHP Version

PHP 8.4.20 (cli) (built: Apr  8 2026 08:16:38) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.4.20, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.20, Copyright (c), by Zend Technologies

Operating System

No response

[devnexen]

Not a bug, expected behavior.

Dom\HTMLDocument::createFromString() runs the HTML5 parser, where namespaces are fixed by the parser, not declared in the markup. Ordinary elements always land in http://www.w3.org/1999/xhtml, only SVG/MathML foreign content differs, and xmlns:* are plain attributes with no namespace meaning in HTML. You can see it: Invoice got lowercased to invoice and the xmlns:* stayed as literal attributes. Browsers do the same.

loadXML() and Dom\XMLDocument use the XML parser, hence the namespace you expect. For XML input, Dom\XMLDocument is the right one, which is why the two classes are separate.