oh. sorry, is answered the wrong question.
you need a username and password for the other webserver, you will need to have the file uploaded onto your server first, like my first answer.
Now, you need to know the correct linux commands for what you want to do. Type this into your php script, enclosed in backticks (`).
eg
ftp server.name -u username -p password
put yourfile /whereyouwant
i say eg, because i havent a clue what the commmands are, im just guessing. Make sure you disconnect from your ftp connection before you finish the script.