There are built in ftp functions. http://us3.php.net/manual/en/ref.ftp.php
. I primarily use the ftp functions for some php scripts I run as cron jobs. Should be able socket functions as well if you are so inclined. I've done that in the past for some basic NNTP functionality.
The upload from the browser to the server is by posting, but you can then FTP the file to an ftp server within the script.