PHP force download png

Hi;
Hi for some reason I cannot use Codeigniters force_download. I thought I learn first how to do it without CI so I can figure it out.

How can I force download this png file? This one downloads a file but the file can not be opened since it's damaged!

PHP Code:

$image='http://clickbank.wpengine.netdna-cdn.com/wp-content/uploads/2014/12/andrea-image-compressed.png';
 if(@getimagesize($image))
 {
 header('Content-type: image/png');
 header('Content-Disposition: attachment; filename=my_file.png');
 readfile(file_get_contents($image));
 } 


I also got this from the manual, but same thing, broken file:

PHP Code:

$image='http://clickbank.wpengine.netdna-cdn.com/wp-content/uploads/2014/12/andrea-image-compressed.png';
header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename='.basename($image));
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($image));
    readfile($image); 


You can achieve that by using CI Download helper.
$this->load->helper('download');
$content = file_get_contents("http://ointuit.ru/wp-content/uploads/2014/09/PHP5.jpg"); // Read the file's contents
$file_name = 'myfile.jpg';

force_download($file_name, $content);