PHP save image

So i have an image object in the php file. How can i make that box appear and let the users save the image in their computer?

I am currently using imagejpeg($img, ””, 100);, which shows the picture and the the user can save it. But i want open that save box auto .. can it be done?

header("Content-Type: image/jpg");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("content-disposition: attachment; filename=my_image.jpg");

imagejpeg($img, null, 100);

thanks!