How do I add elements to an xml document with the php xml dom?

[John C]

For example, given the xml file below:

<?xml version="1.0" encoding="utf-8"?>
<root>
<child>abcdefgh</child>
<secondchild>ijklm</secondchild>
</root>

How would I add an element <thirdchild> after <secondchild> with the php xml dom?