Retrieve the posted values

Dear Community,

its already 2 days that I'm trying to figure out how to do the following task and still now I can't solve the problem!

Please, hope some of you can give me some important tips.

I've an XML file with countries, that I fatch without any problem and I show this countries into a form select.

What I want to do it, post the selected value, get in a result page the posted ID and parse anoter XML file to get the righr detail for i.a. a listing of products.

I can't understand how to get the value in a page set to show the listing.

NOTE.
1) I'm using XML becouse I'm going to connect myself to a web service.

2) when I understand all this, the form submission will be more complex because I'm trying to cereate a booking hotel system, so much more parameters like data etc etc as to be sent.

Please, hope someone can give me a help

thanx you