[ci skip] Add test for GH-12455

This was already indirectly fixed via e127f87.

Co-authored-by: InvisibleSmiley

Author: nielsdos

ext/dom/tests/gh12455.phpt

@@ -0,0 +1,33 @@
+--TEST--
+GH-12455 (Namespace prefixes reused incorrectly depending on libxml2 version)
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$doc = new DOMDocument();
+$element = $doc->createElementNS('http://test', 'a:x');
+$doc->appendChild($element);
+$element1 = $doc->createElementNS('http://test', 'b:y');
+$element->appendChild($element1);
+$element1->appendChild($doc->createElementNS('http://test', 'b:z'));
+echo $doc->saveXml();
+
+$xpath = new DOMXPath($doc);
+$xpath->registerNodeNamespaces = true;
+$xpath->registerNamespace('b', 'http://test');
+
+$elements = $xpath->query('//b:z');
+foreach ($elements as $e) {
+    var_dump($e->nodeName);
+}
+
+$elements = $xpath->query('//*[name()="b:z"]');
+echo $elements->length;
+
+?>
+--EXPECT--
+<?xml version="1.0"?>
+<a:x xmlns:a="http://test"><b:y xmlns:b="http://test"><b:z/></b:y></a:x>
+string(3) "b:z"
+1