ok. we have:
1 - the client, with a file that should be sent somewhere.
2 - your server, where your file chooser script is hosted
3 - the destination server, where the client file should go
then you have to do is:
1 - upload the file from the client to your server (2). for that you need the $_FILES object (because that's where the browser stores the uploaded file, not only the filename, see the upload howto in the PHP manual).
2- if the file is uploaded on your sever, open a ftp connection to the destination server (3) and transfer the client file.
that's it...