Using Global variables in a plugin

I am having a problem with using my own global variables in a plugin. I want to save a value into a global variable in a plugin, and then echo that value in header.php.

In my plugin I declare the global like this at the top of the file:

global $dibbit_value ;

and then set it inside a function like this:

function xyz()
global $dibbit_value ;
$dibbit_value = "test" ;

If I try and echo $dibbit_value in header.php it just comes up blank.

If I set $dibbit_value at the top of my plugin as soon as I declare it, it works ok, but not if I try and set it within a function.

What am I doing wrong?

Thx