This is definitely a BC break, and we can't do that.

I'd argue instead for the following:

- have an optional flag argument to `getConstants()`, `$namesOnly`, that defaults to true. 
- When `$namesOnly` is true, it will proxy to `getConstantNames()`. Additionally, raise an `E_USER_DEPRECATED` error, so that developers know they should alter their code to use `getConstantNames()` if they want the original behavior.
- when the flag is false, do the new behavior.

This will allow developers to opt-in to the new behavior, and give us a migration path.