Maybe I'm being dense, but why not set it to what you want it to be? Clearing it is leaving the decision up to the browser which will not necessarily have the effect you want for all users.
Incidentally, it might not be possible if Apache is setting it. Not sure if PHP has the ability to override headers being sent by Apache.
The two have different effects. The Expires: header proclaims that the page is good for x hours/minutes/days etc. Whereas the browser can use the Last-Modified header to figure the staleness for itself. If its newer than the copy it has, it needs to download the newer copy.
--
Richard Heyes
+44 (0)800 0213 172
http://www.websupportsolutions.co.uk
Knowledge Base and HelpDesk software
that can cut the cost of online support