Cache headers for images

Cache.php

header('Content-Type: image/png');
$modified = $ch->lastmodified();
		if($modified)
		{
		if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $modified)
		{
		header('HTTP/1.0 304 Not Modified'); exit();
		}
		//echo $modified;
		header('Last-Modified: ' .gmdate('D, d M Y H:i:s T', $modified).' GMT');
		}
		header("Cache-Control: max-age=2592000"); //60*60*6 = 6 hours
		header("User-Cache-Control: max-age=2592000");
		header('Expires: '.gmdate('D, d M Y H:i:s \G\M\T', time() + (60 * 60*6))); //6 hours
		header("Pragma: cache");
imagepng($im);
	imagedestroy($im);