Actually, I did get this to work:
Called from plug-in one:
global $my_variable;
$my_variable = 5;
echo "set my_variable to " . $my_variable . "<br>";
Called from plug-in two:
global $my_variable;
echo "read my_variable " . $my_variable . "<br>";
It works. However, this turns out to be not what I really needed.
What I want to do (maybe I should post a new question) is call an external .php module with a click on a button, and somehow have access to variables declared in a plug-in (inside vB's scope).
In other words, a plug-in loads a template and fills it with some buttons, and assigns url links to the buttons. The buttons call a completely separate external php module like:
<a href="http://mydomain.com/forum/external_module.php?dothis=4>
So when the user clicks on a button, a php function in another file is called.
However, I can't seem to figure out how to have access to variables used in the scope of vB when I'm inside my external php code. Maybe this isn't possible... Or maybe (idea) I could hack one of the functions.php files of vB to have my function inside it, then maybe it would be inside the correct scope...have to try that...