Plugin Variables, and Templates Help

[C Braithwaite]

Hiya, i was trying to make a plugin which would display in any template on my site. i simplified the plugin to:

vBulletin
global_start

$mytestvariable = "Hello World";

and then placed the $mytestvariable variable into my template, but it would not show.

Anyone know how i can create a variable that i can use on any page?

Thanks in advance