I am currently using a similar version of the below code to transfer a file from a remote server to my web server and then redirecting to the web server copy of the file in
You can use phpseclib to download the file:
require_once 'Net/SFTP.php';
$connection = new Net_SFTP($remote_server_ip);
if (!$connection->login('username', 'password')) die('Login Error');
// set some appropriate content headers
echo $connection->get($filelink);
Or you can use the ssh2.sftp wrapper - see SilvioQ's answer for that approach.