public static function loginUser($username, $password = null)
 {
     if ($password === NULL) {
         if (Database::doesUserExist($username)) {
             self::setUser($username);
             return true;
         }
         return false;
     }
     if (Database::verifyUser($username, $password)) {
         self::setUser($username);
         return true;
     }
     return false;
 }