error with {vb:raw foruminfo.forumid}

[Reks]

I was looking for a way to get the current forum id to use in the header template now i found somewhere saying that could use this {vb:raw foruminfo.forumid} the problem is that's empty
the code i am trying to make work is this

<script type="text/javascript">
var output;
var url = encodeURIComponent(location.pathname)
output= "<img src='headers/img.php?url="+url+"&id={vb:raw foruminfo.forumid}'>";
document.write(output)
</script>

the problem is that instead of
headers/img.php?url=%2Fforumdisplay.php&id=9
i get
headers/img.php?url=%2Fforumdisplay.php&id=

can anyone help?