How to add a header that distinguishes user's browser , and IE6 is found , tell him to upgrade to Firefox/Chrome/Safari/Opera/IE8 ?

How to write such code in header.php ?
Thanks a lot .

IE6 must die

English is not my native language. I try my best to express my thought precisely. I hope you understand what I mean. If any misunderstanding results from culture gaps, I apologize first.