Code may speak better than words, so here it is

    public static function is_real_user($userid, $checkdb = false) {

        global $DB;

        if ($userid < 0) {

            return false;

        }

        if ($checkdb) {

            return $DB->record_exists('user', array('id' => $userid));

        } else {

            return true;

        }

    }

For $userid = 0, the method will return True if $checkdb is False, else it will return False.

I advocate that it should return False for a userid of 0 as it is not present in the database. This was discovered when I evaluated the possibility of using this method for checking whether a user was meant to be in the database without performing any DB query.