Using a form to write and update a XML file

Hello all.

I was wondering if someone knows how to pass data from a PHP form to update a XML file.

for example: I need a form to upload a picture, generate a thumbnail, write a description ,then get the location data of the picture and thumbnail and so write to an existent XML file like that below

<images>
<Pic>
<image> picture location write form data here </image>
<caption> description text write form data here </caption>
<thumbnail> thumbnail location write form data here </thumbnail>
</pic>
</images>

I need the form to add a node <pic></pic> every time the form is filled.

Can anyone help me?

Thanx,
Wender