newbie: redirecting a POSTed form

Hi, I have to redirect a POSTed form, ie. a form gets posted to my PHP page, and I then do some database stuff and then redirect it to another site. I know of header("http://othersite.com"); to redirect a simple URL request, but how can I redirect a post with a bunch of form fields in it? I don't think I am allowed to use GET for this (it's a credit card authorization provider). Thanks for any tips, I'm a bit baffled by this.. Peter