Add test case for bug #48339

This now works in spec-compliant DOM.

Test by: greatwhitepine at bigfoot dot com

ext/dom/tests/modern/xml/bug48339.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Bug #48339 (DOMElement::setIdAttribute() throws 'Not Found Error' exception)
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$document = DOM\XMLDocument::createEmpty();
+$element = $document->createElement('test');
+$element = $document->appendChild($element);
+$element->setAttribute("xml:id", 1);
+$element->setIdAttribute("xml:id", TRUE);
+echo $document->saveXML(), "\n";
+?>
+--EXPECT--
+<?xml version="1.0" encoding="UTF-8"?>
+<test xml:id="1"/>