client redirection

I think that you can use some sort of $HTTP_REDIRECT method maybe(?!?!?) to redirect the user to a different page. but I understand that this is only before the header has been sent to the client page. What about if you want to redirect the user only if certain conditions exist. What happens then. I have though of using meta data to do this, but can meta data be put anywhere in the page? Doesn't it have to be put in the <head></head> section of the html? Is there any PHP redirect method ie /user_redirect("http://www.whereiwant.com"); this would be pretty good. Maybe I am asking for too much. thanks scott.