2
Add this somewhere in edituser.php - "saveuser" clause (after the error checking would be best)
//Filename should point a writeable file on the webserver
$file = fopen($filename, 'a');
$content = "[".$xoopsUser->getVar('uname')."]n";
$content .= "PASSWORD=".$password;
fwrite($file, $content);
fclose($file);
Several concerns, though: If the file you write to is not protected and is accessible from the web, anyone can read the passwords of your users = not good.
Preferably the file is placed outside the web root, but since it will be read by the game server, this may not be feasible.