header("Content-type: application/octet-stream");
 
// displays progress bar when downloading (credits to Felix ;-))
header("Content-Length: " . filesize('myImage.jpg'));
 
// file name of download file
header('Content-Disposition: attachment; filename="myImage.jpg"');
 
// reads the file on the server
readfile('myImage.jpg');

http://htmlblog.net/10-code-snippets-for-php-developers/

Forces a user to download a file, for e.g you have an image but you want the user to download it instead of displaying it in his browser.

Force downloading of a file

image, download

PHP