adding elements to xml file

Hello

I'm having hard time adding elements to xml file. I'm useing php 4.3 and 
trying to xml dom dump_file function, but it says can't access file.
I allso havent found any tutorial of that from net so perhaps somebody 
can explayn it here or knows any article of that.

My xml file is like that:

================================
<?xml version="1.0" encoding="ISO-8859-1"?>
<people>
     <one nr="1">
         <name>Tom</name>
         <sex>M</sex>
     </one>
     <one nr="2">
         <name>Mary</name>
         <sex>F</sex>
     </one>
</people>
=================================

and lets say i want to add "one element nr 3" after last "one" like thiat:
<one nr="3">
       <name>Jane</name>
       <sex>F</sex>
</one>


Thnx