How do I get the page to automaticly refresh to a url based on the subject

i.e.
if

$POST["subject"] = "Join the Mailing List"

goto

reciept.php?what=Thank%20you%20for%20joining

after 3 seconds

if not then goto

reciept.php?what=Sorry%20to%20see%20you%20leave

after 3 seconds

it is relivent to my last post