Edit XML Files with PHP

Hi,
I just get out how to parse XML Files with PHP. I want tu use a XML-Database and for that I have to modify the data in my XML File and Append "rows".
How do I do that with PHP?

Are there any XML-Functions or do I have to use the normal textfile-functions (read out the complete XML-File, modify the values or append values and save)?

Thx.
Greetz, piz.