PHp Help

I have yet to get a PHP book but soon.

Anyway I have a file called chat.php

In that file I have a variable called $activechatusers

How do I take what is in that variable and make it available in index.php or even better just make it available directly in one of the templates?

Putting

require("chat.php");

at the top of index.php should do the trick :)
Just make sure both files are in the same directory.