Advanced flash gallery with tumbnails

Hi, I just performed the tuturial in the site of creating a photo gallery with thumbnails (great guide). I want to add a feature:

Whenever a user will view a big picture (loaded with XML) he/she can click a button next to it that will send the URL of the picture as a variable to a php file.

Can someone instruct me on which code I need to add (AS2.0) so that the URL of a big pic will be sent to a a var?

Thanks