More robust environment variable fetching

[GrahamCampbell]

Goal

The getenv and putenv functions in PHP are not threadsafe. For this reason, sometimes people choose not to call them when loading any environment variables in a scripted fashion, for example, from a ruby-style .env file, using some custom software.

Thus, we may need to check beyond getenv to find the environment variable the user has set. Fortunately, there is a mature package for doing this (and for loading .env files... it is actually exactly the package that ships with Laravel): vlucas/phpdotenv.

Design

We replace the calls to getenv with checking all of the possible sources environment variables could be in: $_SERVER[...], $_ENV[...], getenv(...), apache_getenv(...) (where available).

Changeset

There area no API changes.

Tests

An additional test has been added, demonstrating that loading the api key and server url into the env superglobal can now be detected by the client on instantiation.

Review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[tomlongridge]

LGTM - thanks @GrahamCampbell!