In a myriad of ways, to be honest. I'm planning on many uses but if a single example is a necessity, I've got a member map that I'm including on certain pages via phpinclude in the template and that included file requires the variable for nearby members and points of interest.
Not without a crystal ball
You can't, only html files.
You can include username, just depends on what page you're on and what template event you're working with.Can $user->data['username'] be passed to a PHP file included via the template without requiring more information
And that actually works for you?