Estoy tratando de lograr lo siguiente:

<?xml version="1.0"?>
<books>
<book>
<name>Harry potter</name>
<category>Adventure | Family | Fantasy</category>
<pages>850</pages>
<author>
<author_name>Jhon Doe</author_name>
<author_wiki>http://wikipedia....</author_wiki>
</author>
<description>lorem ipsum blabla</description>
</book>
</books>

La parte que no puedo poner a trabajar es el elemento de autor intermedio. Pero no puedo ir más allá, entonces, he intentado muchas cosas, pero parece que solo me da páginas en blanco. Lo que tengo ahora:

<?xml version="1.0"?>
<books>
<book>
<name>Harry potter</name>
<category>Adventure | Family | Fantasy</category>
<pages>850</pages>
<description>lorem ipsum blabla</description>
</book>
</books>

<?php header('Content-Type: text/xml;'); 
// Start XML file, create parent node
$doc = new DOMDocument('1.0');
$root = $doc->createElement('books');
$root = $doc->appendChild($root);
// we want a nice output
$doc->formatOutput = true;
$user = $doc->createElement('book');
$user = $doc->appendChild($user);
$title = $doc->createElement('name');
$title = $user->appendChild($title);
$text = $doc->createTextNode('Harry potter');
$text = $title->appendChild($text);
$title = $doc->createElement('category');
$title = $user->appendChild($title);
$text = $doc->createTextNode('Adventure | Family | Fantasy');
$text = $title->appendChild($text);
$title = $doc->createElement('pages');
$title = $user->appendChild($title);
$text = $doc->createTextNode('850');
$text = $title->appendChild($text);
$title = $doc->createElement('description');
$title = $user->appendChild($title);
$text = $doc->createTextNode('lorem ipsum blabla');
$text = $title->appendChild($text);
$user = $root->appendChild($user);
echo $doc->saveXML();
?>