PHP: output very slowly

I want to imitate a very slow server.

How can I make PHP send output very slowly?  I want it to send a few bytes then sleep then send a few more bytes.

<?php 

$image = 'image.png';

header('HTTP/1.1 200 OK', true);
header('Content-Length: ' . filesize($image) , true);
header('Content-Type: image/png', true);
header('Content-Disposition: inline; filename="image1.png"', true);
header('X-Powered-By: XYZ Digital Asset Management', true);
header("Cache-Control: no-cache, must-revalidate", true);
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT", true);
header('Cache-Control: private', true);

readfile($image);

exit;

?>

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