variables in php files

[Zaiaku]

Here;s the one for the last few threads:

$Zforum = 5;

    private function fetch_lastids()
    {
        $lastx = $this->registry->db->query_first("
        SELECT COUNT(`threadid`) AS `lastx`
        FROM " . TABLE_PREFIX . "thread AS thread
        WHERE thread.forumid IN (0,". $Zforum. ")
        AND thread.visible = 1
        ");
        $this->threadscount = $lastx['lastx'];
    }

Now where it says $Zforum in the SQL quiery. If I just replace that with the number 5 then it works.