PHP simpleXML: Read XML, add node and save - php

Question

I'm having some problems with adding a xml node to its parent.
I receive variables $cat, $title and $isbn.
I want to parse $title and $isbn to an XML node and add it to the right categorie ($cat).
die(var_dump($parent)); --> returns NULL, so the biggest problem (I think) is that I can't figure out how to add my node to the right parent because I cant get it identified.
Any suggestions?
The XML file:
<?xml version="1.0"?>
<books version="1.0">
<categorie name="catone" id="100">
<book title="1_WS2012" isbn="isbnone" />
<book title="1W2012DHCP" isbn="ibsntwo" />
</categorie>
<categorie title="cattwo" id="101">
<book title="2W2008R2DC" isbn="isbnthree" />
</categorie>
<categorie title="catthree" id="103">
<book title="3SBS" isbn="isbnfout=" />
</categorie>
</books>
The Code:
//Get variables
$cat = "catone";
$title = "testtitle";
$isbn = "testisbn";
$xmlDoc = simplexml_load_file("books.xml");
$parent = null;
//Construct node
$childstring = "<book></book>";
$child = new SimpleXMLElement($childstring);
$child->addAttribute('title', $title);
$child->addAttribute('isbn', $isbn);
//This works (results in <book title="testtile" isbn="testisbn" />)
//Add node to correct parent
for ($i=0; $i <= sizeof($xmlDoc->categorie) -1; $i++) {
//The condition does also work
if (strtoupper($xmlDoc->categorie[$i]->attributes()->name) == strtoupper($cat))
{
//I'm stuck here
$parent = $xmlDoc->categorie[$i]->attributes()->xpath('/object/data[#type="me"]');;
$xmlDoc->$parent->addChild($child);
}
}
//Write file
file_put_contents("books.xml", $xmlDoc->asXML());
Desired result:
<books version="1.0">
<categorie name="catone" id="100">
<book title="1_WS2012" isbn="isbnone" />
<book title="1W2012DHCP" isbn="ibsntwo" />
<book title="testtitle" isbn"testisbn" />
</categorie>
<categorie title="cattwo" id="101">
<book title="2W2008R2DC" isbn="isbnthree" />
</categorie>
<categorie title="catthree" id="103">
<book title="3SBS" isbn="isbnfout=" />
</categorie>
</books>

Answer 1

First, use xpath to select the parent. xpath is like SQL for XML:
$xml = simplexml_load_string($x); // assume XML in $x
$parent = $xml->xpath("/books/categorie[#name = 'catone']")[0];
Note: The above code requires PHP >= 5.4 for the [0] at the end of line 2. (1)
Now, add the new <book> and its attributes:
$new = $parent->addChild("book","");
$new->addAttribute("title", "testtitle");
$new->addAttribute("isbn", "testisbn");
see it working: https://eval.in/131009
(1) if you are on PHP < 5.4, either update or do:
$parent = $xml->xpath("/books/categorie[#name = 'catone']");
$parent = $parent[0];