getenv vs $_ENV

[jeromegamez]

Is your feature request related to a problem? Please describe.
getenv() and putenv are not thread-safe - unfortunately, I can't seem to find a reliable official source right now, but vlucas/dotenv and a Symfony discussion mention this.

Describe the solution you'd like
I'm wondering if it might make sense to use the $_ENV superglobal to get and set environment variables, for example with $value = getenv($key) ?: $_ENV[$key]; (simplified code to show my point).