How to cache images generated by PHP

When spitting out images with php the browser doesn't seem to cache the resulting image because the browser was no caching.

This image script pulled off 400mb in 3 days:
http://ruralromeos.com/new_singles.php

To fix this we simply told the browser to cache the image.

There are two options in the code below, use whichever one suits you. (ie: if you know the image will never change use option 1, or if you have a file to base the mtime off use option 2).

session_start(); // if you are using sessions.
// not sure if this stuff is really needed, but works:
header("Cache-Control: private, max-age=10800, pre-check=10800");
header("Pragma: private");
header("Expires: " . date(DATE_RFC822,strtotime(" 2 day")));
// the browser will send a $_SERVER['HTTP_IF_MODIFIED_SINCE'] 
// option 1, you can just check if the browser is sendin this
if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])){
  // if the browser has a cached version of this image, send 304
  header('Last-Modified: '.$_SERVER['HTTP_IF_MODIFIED_SINCE'],true,304);
  exit;
}
// option 2, if you have a file to base your mod date off:
$img = "some_image.png";
if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) 
       && 
  (strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == filemtime($img))) {
  // send the last mod time of the file back
  header('Last-Modified: '.gmdate('D, d M Y H:i:s', filemtime($img)).' GMT', 
  true, 304);
  exit;
}

// and here we send the image to the browse with all the stuff
// required for tell it to cache
header("Content-type: image/jpeg");
header('Last-Modified: ' . gmdate('D, d M Y H:i:s', filemtime($img)) . ' GMT');
// use ob buffering to find the image size.
ob_start();
imagejpeg($image,null,100);
$ImageData = ob_get_contents();
$ImageDataLength = ob_get_length();
ob_end_clean();
header("Content-Length: ".$ImageDataLength);
echo $ImageData;
imagedestroy($image);