Now supporting if-modified-since !

For quite a while, I've been meaning to add a full-text RSS feed to my site. There's one obstacle, though: Since I instated it, the RSS feed was the top bandwidth hog on my site. A full-text feed would mean that, every time someone's RSS reader checks whether I've written something new on this site, it would download all articles. I simply didn't want to take that hit, especially in light of the upcoming new version of the Moose, which tends to eat bandwidth like there's no tomorrow. But then, in the feature description of one blogging engine, I read that HTTP supports just sending back a "this page is unchanged" reply. So, I added that. Read on to learn how:

The first thing every sensible programmer will do when faced with something new is to google it. So I did, and apart from RFC 2616 "HTTP 1.1", it brought up a forum entry at WebmasterWorld containing some nice sample code, and I also remembered PHP's header function, which has user-comments that show how to generate the proper reply.

So, how does it work? Glad you asked! When a browser requests your web page, it can send either a regular GET request, or if it has a cached copy of your page, it can send a conditional GET request instead. The difference between the two is simply that the conditional request contains an additional If-Modified-Since: header that contains the date of the cached copy. All your code has to do is compare that date to the change date of the page you're about to display. If they don't match, send back a 304 not modified "error" and don't send the page. That will tell the browser it's okay to show the cached response.

If your page has changed, just send the page like you used to do. Of course, to make sure the browser has the correct data, you should also send along a Last-Modified: header with every successful request. That's all. The code to do this that I'm using is:

$cond = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : 0;
    
if( $cond && ($_SERVER['REQUEST_METHOD'] == 'GET')
  && (strtotime( $cond ) >= $aw_newestdate) )
{
    header( "HTTP/1.0 304 Not Modified" );
    exit;
}

header( "Last-Modified: " . gmdate( "D, d M Y H:i:s", $aw_newestdate ) . " GMT" );

// echo page contents here.