I think, that the problem is not to list files on the client side (wouldn't work with PHP, as cou wrote), but the problem is that the Server would not know how to handle an incoming filepacket.
There is a company, that offers a product (or actually 2), that does exactly what you need. You can find it at http://www.persits.com/. It's ASP however... :-(
I wrote them if it's possible with PHP too, I'll post the answer here, when received.
Regards,
Matthias