Oh I was thinking of adding the custom CRBN or Carb or whatever as class aliases to the original CarbonImmutable class in config/app.php. I actually haven't used aliases yet though, so I'm going by the assumption that they "just work".

I've had problems in other projects too where I wanted to disallow use of classes like Carbon after declaring a child class that overrides certain behaviors, so that the code is more future-proof for newly onboarded developers who don't know the pitfalls yet. In C/C++ I would just do:

  #define Carbon "Please don't use Carbon, use CRBN or CRBNMUT instead."

So attempts to use Carbon would show up as compiler errors. But there are no #defines in PHP, so I haven't figured out a way to do that yet :-/ Maybe I could declare Carbon as a constant somewhere or something. But I don't think there's a way to undefine constants for times when the original word is needed, like with #undef.