Your eyes are fine. You need to check for If-Modified-Since header, if the time is older than file modification time (filemtime()) send Last-Modified header and the image, else send 304 Not Modified response.
This code seems to work. Have I got it right?
// Get the time the cache file was last modified
$lastModified = filemtime($pPath);
// Issue an HTTP last modified header
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', $lastModified) . ' GMT');
if (isset($_GET['If-Modified-Since']))
{
// Split the If-Modified-Since (Netscape < v6 gets this wrong)
$modifiedSince = explode(';', $_GET['If-Modified-Since']);
// Turn the client request If-Modified-Since into a timestamp
$modifiedSince = strtotime($modifiedSince[0]);
}
else
{
$modifiedSince = 0;
}
// Compare time the content was last modified with client cache
if ($lastModified <= $modifiedSince)
{
header('HTTP/1.1 304 Not Modified');
}
else
{
$extention = substr($path, -3);
if ($extention == "jpg")
header("Content-type: image/jpeg");
if ($extention == "gif")
header("Content-type: image/gif");
if ($extention == "bmp")
header("Content-type: image/bmp");
if ($extention == "png")
header("Content-type: image/png");
readfile("$pPath");
}