Example of date-based browser caching

If-Modified-Since.php

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= filemtime($path_to_image)) {
    header('HTTP/1.0 304 Not Modified');
    header('Cache-Control: public, max-age=2592000');
    header('Last-Modified: Mon, 27 Oct 2014 17:08:41 GMT');
    exit(0);
} else {
    header('Content-type: image/jpeg');
    header('Cache-Control: public, max-age=2592000');
    header('Last-Modified: Mon, 27 Oct 2014 17:08:41 GMT');
    echo file_get_contents($path_to_image);
}