Hi,
That is because the script you refer to is javascript. Most vars you can easily get through the variables parsed by the browser. you cannot get screen settings, and hardware settings this way though.
the problem is that PhP is server-side. That means that your script will never get to the client (surfers) computer. Therefor you are dependent on the indo the clients browser gives the server. And that does not contain information of the hardware of the clients system.
Javascript on the other hand is client side, and can get some more info. ALthough I doubt you can get HD etcetc...
If you want to do this in php have a look at:
$REMOTE_ADDR
and search a bit for the operating system scripts. THere are load of them on the web.
J.