304: Not modified and front end caching

Question

I am using a PHP script to serve files. I would like to be able to send back a 304 not modified header in my http response if the file has not changed since th

Answer 1

$_SERVER['HTTP_IF_MODIFIED_SINCE'] is usually empty when register_globals is off.

Check whether that's the case, and if so try getenv('HTTP_IF_MODIFIED_SINCE')

Answer 2

There are also some others parameters to check .. in my case I didn't had both of those headers :

$_SERVER['HTTP_IF_NONE_MATCH'] && $_SERVER['HTTP_IF_MODIFIED_SINCE']

which are required to return a proper 304 header, as my system clock was a little late, It'll interpret those pages as expiring in the future, then not sending those values at all

Also check that header which is returned by apache, or at least override it to a bigger value

Cache-Control: max-age=3600

As it won't send previous headers if

Last-Modified previous sent header < ( NOW - 3600 )

So in my case I've set this pretty handy function function lastModified($file){ $x=filemtime($file); while($x>time())$x-=86000;}#reduce by one day if touched in future date $date=gmdate('D, j M Y H:i:s',$x).' GMT'; header('Cache-Control: max-age=86000',1); if($_SERVER['HTTP_IF_NONE_MATCH'] == $x || $_SERVER['HTTP_IF_MODIFIED_SINCE']==$date){ header('HTTP/1.1 304 Not Modified',1,304);die;} header('Etag: '.$x,1);header('Last-Modified: '.$date,1); }

Answer 3

Note that $_SERVER["HTTP_IF_NONE_MATCH"] can contain quotes and -gzip suffix.

$server_etag = str_replace("-gzip", "", str_replace('"', '', stripslashes($_SERVER['HTTP_IF_NONE_MATCH'])));

if ($server_etag == $etag) ...

Answer 4

This article will answer all your questions on caching

I found that adding

RewriteRule .* - [E=HTTP_IF_MODIFIED_SINCE:%{HTTP:If-Modified-Since}]
RewriteRule .* - [E=HTTP_IF_NONE_MATCH:%{HTTP:If-None-Match}]

To the bottom of my htaccess file (below all rewriterule) worked.

Answer 5

HTTP_IF_MODIFIED_SINCE is the right way to do it. If you aren't getting it, check that Apache has mod_expires and mod_headers enabled and working properly. Borrowed from a comment on PHP.net:

$last_modified_time = filemtime($file); 
$etag = md5_file($file);
// always send headers
header("Last-Modified: ".gmdate("D, d M Y H:i:s", $last_modified_time)." GMT"); 
header("Etag: $etag"); 
// exit if not modified
if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == $last_modified_time || 
    @trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag) { 
    header("HTTP/1.1 304 Not Modified"); 
    exit; 
}

// output data

Answer 6

I had this problem and it turned out to simply be that I had Firebug open. This has an option under the Net tab "Disable Browser Cache" that is ticked by default. There is a similar option in Chrome's developer tools, one of the tick boxes on the bar under the menu bar.

Unticking these options resulted in the browser correctly sending HTTP_IF_MODIFIED_SINCE and everything working fine after all (even with Firebug or Chrome Dev Tools open).