Define cloneDocument for XSLTProcessor, and test the property

This was introduced in 5c039bb, but never defined on the stub.
It was more or less fine when dynamic properties were not deprecated,
but now they throw a deprecation warning. To fix it, define on the stub.
This should also help discoverability of the functionality.

Author: nielsdos

ext/xsl/php_xsl.stub.php

@@ -74,6 +74,9 @@ class XSLTProcessor
     /** @var bool */
     public $doXInclude;
 
+    /** @var bool */
+    public $cloneDocument;
+
     /**
      * @param DOMDocument|SimpleXMLElement $stylesheet
      * @tentative-return-type

ext/xsl/php_xsl_arginfo.h

@@ -0,0 +1,45 @@
+--TEST--
+cloneDocument
+--EXTENSIONS--
+xsl
+dom
+--FILE--
+<?php
+
+$xml = new DOMDocument;
+$xml->loadXML('<?xml version="1.0"?><root><foo>hello</foo></root>');
+
+function test() {
+    global $xml;
+    $xml->documentElement->firstChild->textContent = "bye";
+}
+
+$xsl = new DOMDocument;
+$xsl->loadXML(<<<XML
+<?xml version="1.0"?>
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:php="http://php.net/xsl" version="1.0">
+    <xsl:template match="/root">
+        <xsl:value-of select="php:function('test')"/>
+        <xsl:value-of select="//root/foo"/>
+    </xsl:template>
+</xsl:stylesheet>
+XML);
+
+$xslt = new XSLTProcessor;
+$xslt->registerPHPFunctions();
+$xslt->cloneDocument = true;
+$xslt->importStylesheet($xsl);
+echo $xslt->transformToXml($xml);
+
+$xslt = new XSLTProcessor;
+$xslt->registerPHPFunctions();
+$xslt->cloneDocument = false;
+$xslt->importStylesheet($xsl);
+echo $xslt->transformToXml($xml);
+
+?>
+--EXPECT--
+<?xml version="1.0"?>
+hello
+<?xml version="1.0"?>
+bye