The "If-Modified-Since" header is something the browser sends to the server, to indicate it's got a copy from such-and-such a date. That gives the server the opportunity to tell the browser that the old copy is still current.
This would not be a header you would add to your script...or any page being send *from* to server.
It is primarily intended for static files. While there could be some useful applications for considering the browser's If-Modified-Since request in a dynamic script, I doubt it would be fruitful for most typical dynamic pages.
ps. To get a comprehensive understading of the HTTP protocol, I can't recommend enough the book "Web Client Programming with Perl" by Clinton Wong [O'reilly, 1997...$5 used on Amazon] (even if you don't know perl) and the application HTTP Tracer ($25 from http://www.lazydogutilities.com/) . You'd be *amazed* how HTTP Tracer can help you cut to the quick of a problem when nothing else is making any sense.
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Sent by Joe Goldsmith on 04/06/2004 08:55:19 PM:
For page rankings the Google Bot wants if-modified since added to each page. At http://www.google.com/webmasters/guidelines.html it states:
How would I add this to each page using FoxWeb?