Thanks - I'll look into that. I think rdiff might be closer to what I want because what I need to be able to do is see whether or not any changes have been made to the files on the remote server before I overwrite them, so that I can sync my local copy of the files before I overwrite the copies on the live server. If rsync or rdiff can compare the file checksums and ask me whether I want the files overwritten that would probably work. What I was hoping for was a networked version of diff, so maybe I could do something like
diff /htdocs/my_web_site/my_script.php ftp://username:firstname.lastname@example.org/my_script.php
and diff would go and get the file on the remote server for me. Maybe I could do what I want using PHP - loop through all the files and retrieve them via FTP, then pipe the output to diff. Not exactly optimal but it might work.