[RFC] [4.0] Behaviour of nested variables

[GrahamCampbell]

There is no way to write a dollar sign without phpdotenv trying to evaluate it as a variable. This is fine, so long as you don't actually have a variable of that name, but will cause problems otherwise. Should we:

1. Move evaluation of nested variables to the parser, or at least have some more intelligent way of the parser to produce output so that its clear what variables are fair game for replacement.
2. Only allow variable evaluation within double quoted strings?
3. Add a way to prevent variable evaluation with an escape sequence?

Both 2 and 3 depend on 1.

[GrahamCampbell]

Should we aim for closer feature parity with bash and/or docker env files?