Caching image requests through PHP - If-Modified-Since not being sent

Question

I am serving images through php and having some problems setting it up to respond with 304 headers to save on load time.

Most of the code below I found on php.net. I

Answer 1

I believe it should be

if (isset($headers['If-Modified-Since']) && (strtotime($headers['If-Modified-Since']) >= $file_time)) {

Checking if the modified time is greater than or equal rather than just equal. Although I do understand the two values should be the same.

Answer 2

mandor at mandor dot net posted a solution at the PHP.net documentation for the header function which worked for me:

<?php

        // Test image.
        $fn = '/test/foo.png';

        // Getting headers sent by the client.
        $headers = apache_request_headers();

        // Checking if the client is validating his cache and if it is current.
        if (isset($headers['If-Modified-Since']) && (strtotime($headers['If-Modified-Since']) == filemtime($fn))) {
            // Client's cache IS current, so we just respond '304 Not Modified'.
            header('Last-Modified: '.gmdate('D, d M Y H:i:s', filemtime($fn)).' GMT', true, 304);
        } else {
            // Image not cached or cache outdated, we respond '200 OK' and output the image.
            header('Last-Modified: '.gmdate('D, d M Y H:i:s', filemtime($fn)).' GMT', true, 200);
            header('Content-Length: '.filesize($fn));
            header('Content-Type: image/png');
            print file_get_contents($fn);
        }

    ?>

Answer 3

Check if sessions are being used on that page, If so, try this:

session_cache_limiter(false);

If the above worked, here's the explanation:

Php's session mechanism sends some automatic cache-related headers in order to improve the session cookie privacy, avoiding it to be cached by intermediate proxyes:

http://php.net/manual/en/function.session-cache-limiter.php

These automatic headers cause the browser not to ever send the If-Modified-Since header, as they instruct it to not to perform any caching at all.

Answer 4

After searching for a while, I've found the answer. The browser didn't cached anything (and didn't send the If-Modified-Since) until I sent the following header:

Cache-Control: private;

After doing this all worked fine.

Answer 5

I had to use Keith's solution with combination of azkotoki's and Zsolti's posts above to make everything work as required.

so, final example would be:

<?php

    // Test image.
    $fn = '/test/foo.png';

    session_cache_limiter(false);
    header('Cache-Control: private');

    // Getting headers sent by the client.
    $headers = apache_request_headers();

    // Checking if the client is validating his cache and if it is current.
    if (isset($headers['If-Modified-Since']) && (strtotime($headers['If-Modified-Since']) == filemtime($fn))) {
        // Client's cache IS current, so we just respond '304 Not Modified'.
        header('Last-Modified: '.gmdate('D, d M Y H:i:s', filemtime($fn)).' GMT', true, 304);
    } else {
        // Image not cached or cache outdated, we respond '200 OK' and output the image.
        header('Last-Modified: '.gmdate('D, d M Y H:i:s', filemtime($fn)).' GMT', true, 200);
        header('Content-Length: '.filesize($fn));
        header('Content-Type: image/png');
        print file_get_contents($fn);
    }

?>