How to force image download in php by modifying header, _PHP tutorial

function downloadFile($file){
 $file_name = $file;
 $mime = 'application/force-download';
 header('Pragma: public'); // required
 header('Expires: 0'); // no cache
 header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
 header('Cache-Control: private',false);
 header('Content-Type: '.$mime);
 header('Content-Disposition: attachment; filename="'.basename($file_name).'"');
 header('Content-Transfer-Encoding: binary');
 header('Connection: close');
 readfile($file_name); // push it out
 exit();
}