curl filetime with dynamic images

I have a script that uses curl to check the modified time of a file on a remote server.

<?

$curl = curl_init("http://perfectweb.com/experts/curl_time/oh_noes.jpg");
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_NOBODY, true);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_FILETIME, true);
$result = curl_exec($curl);

if ($result === false) {
	die (curl_error($curl)); 
}

if (curl_error($curl))  echo curl_error($curl);
		
$timestamp = curl_getinfo($curl, CURLINFO_FILETIME);
if ($timestamp != -1) { //otherwise unknown
	echo date("Y-m-d H:i:s", $timestamp); //etc
}

?>

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It works great if I point it to a jpg on my server, as shown above. Now I want to combine it with a dynamic image script that looks like this:

<?

## eventually this script will deliver different images, but one image is hard coded for now
$filename = 'oh_noes.jpg';

header("Content-type: image/jpg");
header("Content-Disposition: attachment; filename=\"".basename($filename)."\"");
readfile("$filename") or display_error('The file could not be found.');
exit;

?>

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(I have this script saved with a ".jpg" extension and forced to run as PHP with htaccess.)

The image script works great when called directly, but now I want to combine the two. If I try to use the file time script with the dynamic image script, it doesn't work. How can I make CURLOPT_FILETIME work with a jpg delivered by a PHP script? Is there a way to detect an incoming CURLOPT_FILETIME request and send the right headers to make curl happy?