标头缓存控件不起作用标头“最后修改”

Question

I make an Image with php and would like to control the cache-time. I have this code:

header("Cache-Control: must-revalidate"); 

$fn = gmdate('D, d M Y H:i:s \G\M\T', time() + 60);
$now = gmdate('D, d M Y H:i:s \G\M\T', time());

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) &&
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) <= $now  )
{
        // Client's cache IS current, so we just respond '304 Not Modified'.
        header('Last-Modified: '.$fn, true, 304);
}else {
        // Image not cached or cache outdated, we respond '200 OK' and output the image.
        header('Last-Modified: '.$fn, true, 200);
        //Header
        header("Content-type: image/PNG");
        //Ausgeben
        imagePNG($bild);

};

It should give a new image only after 60sec. But my code gives it always.

Answer 1

I think something with your arithmetic is off; Look at the following example based on your code:

$lifetime = 60;

header("Cache-Control: must-revalidate");

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
  $lastMod = strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']);
} else {
  $lastMod = 0;
}

if ($lastMod <= $_SERVER['REQUEST_TIME'] - $lifetime) {
  // Time to refresh
  $lastMod = $_SERVER['REQUEST_TIME'];
  header("Content-type: text/plain");
  header('Last-Modified: ' . gmdate('D, d M Y H:i:s \G\M\T', $lastMod), true, 200);
  echo "Hello!";

} else {
  header("Last-Modified: " . gmdate('D, d M Y H:i:s \G\M\T', $lastMod), true, 304);
}

This will set the last-modified header to now (using $_SERVER['REQUEST_TIME'] which is likely much more efficient for your need than using time() directly), and on subsequent requests check if If-Modified-Since is at least 60 seconds old. If so, it will refresh (and re-set last-modified to now); otherwise, 304 is returned and last-modified is not changed.