Reduced by 27 characters or so

Shortened it quite a bit by:
* Make the function non-static. It means the user has to make an instance, but you save 14 characters.
* Use $_ENV instead of getenv(). Saves 2 characters.
* Use concatenation of keys rather than a two-dimensional array. Saves 2 characters.

This kind of changes the structure by making it an instantiated class rather than static, so feel free to reject this.