$url = "https://jira.account/secure/attachment/".$attachment_id."/".$attachment_name."?os_username=". $username ."&os_password=" . $password;

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);

$file = curl_exec($ch);
curl_close($ch);

if($file){
 header('Content-Description: File Transfer');
 header('Content-Type: application/zip');
 header('Content-Disposition: attachment; filename='.$attachment_name);
 header('Expires: 0');
 header('Cache-Control: must-revalidate');
 header('Pragma: public');
 echo ($file);
 exit;
}else{
 throw new Exception("Error:  file now found.");
}