How to code my HTTP response header for 8 hours in the future in php/

[bcomtr]

I know how to set a basic expires HTTP response header in PHP as follows...\[code\]header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");\[/code\]However, I want to make it a little more dynamic in that I want to, using PHP, specify an expiration time eight hours ahead of when the user accesses the content. Could someone help me achieve this?Thanks in advance!