304 Not Modified issue

Question

Sorry for the probably wrong title. I am writing some code to handle If-Modified-Since and If-None-Match requests as part of caching. Everything works perfect except for tha

Answer 1

Try this code:

$last_modified = filemtime($f);
if(isset($_SERVER["HTTP_IF_MODIFIED_SINCE"])) {
    $expected_modified = strtotime(preg_replace('/;.*$/','',$_SERVER["HTTP_IF_MODIFIED_SINCE"]));
    if($last_modified <= $expected_modified) {
        header("HTTP/1.0 304 Not Modified");
        exit;
    }
}

Answer 2

Finally solved this bug. Gzip was the culprit. Since I was gzipping the responses to If-Modified-Since and If-None-Match requests too, gzip was adding a few bytes (kind of a gzip header) to the response. Now I have stopped gzipping responses to If-Modified-Since and If-None-Match requests, and it works like a charm.

Answer 3

I found the solution in Answering HTTP_IF_MODIFIED_SINCE and HTTP_IF_NONE_MATCH in PHP

Create new file caching_headers.php

<?php
function caching_headers($file,$timestamp){
    $gmt_mtime=gmdate('r', $timestamp);
    header('ETag: "'.md5($timestamp.$file).'"');
    if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])||isset($_SERVER['HTTP_IF_NONE_MATCH'])){
        if ($_SERVER['HTTP_IF_MODIFIED_SINCE']==$gmt_mtime||str_replace('"','',stripslashes($_SERVER['HTTP_IF_NONE_MATCH']))==md5($timestamp.$file)){
            header('HTTP/1.1 304 Not Modified');
            exit();
        }
    }
    header('Last-Modified: '.$gmt_mtime);
    header('Cache-Control: public');
}
?>

and add this in all php files that you would like to cache:

<?php
include('caching_headers.php');
caching_headers($_SERVER['SCRIPT_FILENAME'],filemtime($_SERVER['SCRIPT_FILENAME']));
?>