There is no way to be sure which file is the most recent, as there is no such thing as an "uploaded time" attribute. You didn't mention much about the FTP server, but if you have some level of management over the uploads, you could ensure that the last modified time is set on upload. Whether this ends up working is down to your FTP server and possibly clients.
Assuming your modified times are equivalent to upload times, you could do something like this:
// connect
$conn = ftp_connect('ftp.addr.com');
ftp_login($conn, 'user', 'pass');
// get list of files on given path
$files = ftp_nlist($conn, '');
$mostRecent = array(
'time' => 0,
'file' => null
);
foreach ($files as $file) {
// get the last modified time for the file
$time = ftp_mdtm($conn, $file);
if ($time > $mostRecent['time']) {
// this file is the most recent so far
$mostRecent['time'] = $time;
$mostRecent['file'] = $file;
}
}
ftp_get($conn, "target.txt", $mostRecent['file'], FTP_ASCII);
ftp_close($conn);