what are the default http headers (esp last-modified)

[ben]

Hi.

I'm assuming caches use the HTTP header Last-Modified date to determine rules for caching content, I'm interested to know what PHP sends as the default Last-Modified header.

Specifically, if the Last-Modified header is set to the current date/time, wouldn't that mean that most caches would never cache average PHP content (unless you set the HTTP headers to something more useful)?

Any clues on this or sites to check out would be great.

Also, if anyone knows an easy way to see in your page what the http response headers associated with a page are, I'd be keen to know.

Thanks.

[Weedpacket]

Use fsockopen() to get a file from the web server; when you read it all the http headers will still be attached.

[ben]

thanks for that - I found a mozzilla product that also shows the headers:
http://webtools.mozilla.org/web-sniffer/

I was interested to see that heaps of sites (inc PHP ones) don't set much in the way of headers (eg they don't set a default, they just leave them out!). I then looked into how Squid works and found that it needs these headers for it's calculations or (I'm guessing here) it can't make useful calculations about freshness.

I'm wondering how caches choose how long they hold onto dynamic content without these headers?

Ta again
Ben

[Weedpacket]

How the cache works such things out depends on the cache, the HTTP specification deliberately doesn't go into such detail when it discusses when caches are supposed to expire pages (section 13.2: Expiration Model). It says that servers should send expiration dates, or failing that, Last-Modified headers, but it isn't necessary. Most PHP pages are generated dynamically - it could be said that the page is constantly being modified (a forum, for example, or something that responds to form input). A news page or blog, though, could send a "Last-Modified" header based on the time that the most recent article was added.