Current URL inside Hyperlink

[Speedy2000]

I'm basically attempting to create a 'share to Twitter'-button. It uses this code:

<a href="http://twitter.com/home?status=Currently reading http://www.website.com/postx" title="Click to send this page to Twitter!" target="_blank">Share on Twitter</a>

Now I want the code to automatically insert the current URL (of the current thread) (replacing the www.website.com/postx).

Because I have vbSEO installed, I tried to use the variable $_SERVER[VBSEO_URI] (but also the original vBulletin variable http://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]). But apparently the link doesn't translate any of those variables into actual links. Maybe because the variable is placed inside the quotation marks.

Is there a way to have it properly translate the variable to a usable URL?