You will have to send them using $_GET ... so you attach the variables to his URL,
and he then has to read them. He cannot read your cookies or sessions and even
with $_GET and $_POST, he will need to be able to accept your variables ... so he
defines the variables that you send?
You mentioned using POST only, but forms are really the only way. You would have
to redisplay the form (with the variables in place) and ask the user to submit again
to POST to his site. Otherwise, use fsockopen() to connect with the server and construct the HTTP request yourself ... which is pretty advanced (I don't know much about it).
Who really wants the data, you or him?
Why do both of you need the form data?