浏览器不会缓存动态CSS - 响应返回200 OK而不是304 Not Modified

Question

I have dynamic CSS file like this:

$Css = get_from($_u_6);



/* do stuff and other things here... */



$expires = 60*60*24; 
header("Pragma: public");
header("Cache-Control: maxage=".$expires.", must-revalidate, public");
header('Expires: ' . gmdate('D, d M Y H:i:s', time()+$expires) . ' GMT');
header('Content-type: text/css');


echo $Css;

Headers are set correctly: http://i.imgur.com/MIH7j5U.png

But The server always responds with HTTP 200 OK (and the correct content), rather than with a HTTP 304 ( Not Modified).

How can I convince the browser and the server to cache these dynamic css files and to send 304 response?

Answer 1

The Expires: header tells the browser or cache server how long it can keep reusing the same resource without reloading it from the origin server.

If you want the browser to make a conditional request (eg. using If-Modified-Since: or If-None-Match:), you need to send a Last-Modified: and/or ETag: header, and you need to write code to test for these headers and produce the appropriate response (304 or 200).

See RFC 2616 RFC 7232 for full details.