I'm using Gandi.net's hosting service and quite recently they started doing something alarming. They are caching the output of PHP programs, apparently to avoid running the PHP code, and sending that whenever user(s) at a particular IP address request the page. As a result, I can fetch a page in Firefox e.g. one that prints my user agent, and it looks fine but if I put that URL into Chrome or Safari or even fetch the page with Curl, I will see the output that I saw in Firefox. This runs 180 degrees contrary to how I thought web servers are supposed to operate. After all, PHP is a programming language and it can output different HTML from second to second. Who are they to presume it will always output the same data and cache it? My PHP files have the .php extension by the way.
Is there a way around this caching mechanism e.g. can I tell PHP to put something in the header that will tell the cache to not cache the HTML?