Xml update using php

This code works, but it only replaces the XML already written with new XML, I dont want to replace old XML, I want to add new. Anyone know how I might create new xml, rather than replace?

I have this code:

<?php 


// set name of XML file 
$file = "usethis.xml"; 

// load file 
$xml = simplexml_load_file($file) or die ("Unable to load XML file!"); 

// modify XML data 
$xml->parent->name = $_POST['name']; 
$xml->parent->words = $_POST['words']; 


// write new data to file 
file_put_contents($file, $xml->asXML());

include("usethis.xml");
?>