    public static function is_exist($user) {

        if(isset($_GET['act']) && $_GET['act'] == 'edit'){
            $id = Typo::int($_GET['id']);
            $where = "AND `id` != '{$id}' ";
        }else{
            $where = '';
        }
        $user = sprintf('%s', Typo::cleanX($user));
        $sql = sprintf("SELECT `userid` FROM `user` WHERE `userid` = '%s' %s ", $user, $where);
        $usr = Db::result($sql);
        $n = Db::$num_rows;
        if($n > 0 ){
            return false;
        }else{
            return true;
        }

    }

The software constructs all or part of an SQL command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended SQL command when it is sent to a downstream component.