Handling If-modified-since header in a PHP-script

Question

I have a PHP script which is called with an ?img= parameter.

The value for that parameter is an (urlencoded) URL of an image.

My script chec

Answer 1

I recently had to use this feature (serving image via PHP) in order to make images visible only for registered users. Jack's code was helpful, but I had to do a few hacks for it to work perfectly. Thought I should share this one.

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && 
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= filemtime($path_to_image))
{
    header('HTTP/1.0 304 Not Modified');
    header("Cache-Control: max-age=12096000, public");
    header("Expires: Sat, 26 Jul 2015 05:00:00 GMT");
    header("Pragma: cache");
    exit;
}else{
    header("Content-type: image/jpeg");
    header("Cache-Control: max-age=12096000, public");
    header("Expires: Sat, 26 Jul 2015 05:00:00 GMT");
    header("Pragma: cache");
    echo file_get_contents($path_to_image);
}

In short, the script returns Not Modified if it's a browser request HTTP_IF_MODIFIED_SINCE. Otherwise, the image is served with appropriate headers and expiry dates.

Answer 2

This is definitely possible in PHP!

When the browser checks if there were modifications, it sends an If-Modified-Since header; in PHP this value would be set inside $_SERVER['HTTP_IF_MODIFIED_SINCE'].

To decode the date/time value (encoded using rfc822 I believe), you can just use strtotime(), so:

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && 
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= filemtime($localFileName))
{
    header('HTTP/1.0 304 Not Modified');
    exit;
}

Explanation: if the If-Modified-Since header is sent by the browser AND the date/time is at least the modified date of the file you're serving, you write the "304 Not Modified" header and stop.

Otherwise, the script continues as per normal.