Assigning different value onto __var given existing variables?

How would I make __var have different value per threads using existing variables? I want to test a idea.

I don’t understand the request.
$__var has been specifically introduced to share variable values across threads, so why would you like to have different values per thread for this kind of variables ?
Why not just using $_var then ?

I was looking into optimization for rep_colmt, and did a few things. Right now, it does parallel processing per channel after creation of image, but my plan to do parallel processing everything at once while creating a new image. One of my idea is to use apc and use different const per threads, hence the need for different values per threads.