Description:
------------
Environment variables with hyphens in their names (e.g: `getenv('A-B')`) are lost when `proc_open` creates a child process (other process launching functions untested).

Given that these appear to be valid (they're supported by 'env' on Ubuntu) and *worked under PHP 5* this seems to be a regression.

Test script:
---------------
https://gist.github.com/PeterJCLaw/23719e515c787e70b614f45392dc21aa

Expected result:
----------------
-- SUBPROCESS START --

A-B=ZZZ
FOO=BAR
getenv("INNER"): aeiou
getenv("INNER-VAR"): aeiou
done
command returned 0

-- SUBPROCESS END --

Actual result:
--------------
-- SUBPROCESS START --

A-B=
FOO=BAR
getenv("INNER"): aeiou
getenv("INNER-VAR"): 
done
command returned 0

-- SUBPROCESS END --