Read from environment when PHP doesn't populate $_ENV

To help to build a local development environment,
play nice with generic PHP configurations in the wild.

On Fedora and Ubuntu, or if copying php-development.ini
into php.ini, $_ENV array isn't populated.

This issue has been has been solved at keruald/omnitools level,
so we can reuse it for other applications too, reading from
the $_SERVER superglobals array as fallback.

See also https://devcentral.nasqueron.org/D2497 for more context.