This might be the cause of the issue of the empty $_ENV. Quote from (my) default /etc/php.ini (php 8.0 on Fedora 35)

; This directive determines which super global arrays are registered when PHP
; starts up. G,P,C,E & S are abbreviations for the following respective super
; globals: GET, POST, COOKIE, ENV and SERVER. There is a performance penalty
; paid for the registration of these arrays and because ENV is not as commonly
; used as the others, ENV is not recommended on productions servers. You
; can still get access to the environment variables through getenv() should you
; need to.
; Default Value: "EGPCS"
; Development Value: "GPCS"
; Production Value: "GPCS";
; http://php.net/variables-order
variables_order = "GPCS"

So this might even effect other tools, not only the build-in webserver. For me php -f worked, where the build-in webserver did not with the same files, due to unpopulated $_ENV (E)

Thanks for the quick commit f608f0b @nicolas-grekas but I do not think this solves the issue. e.g. 'PATH' is neither in $_SERVER nor in $_ENV for me, only in getenv() (Server is not empty, as ENV is, but does not hold the PATH variable by [my] default)

Is that a guess or did you give it a try? To me it should fix the issue... :)

I reimplemented it, you are right, it works like a charm. I did not expect that the intersection is empty. Thanks for the quick response!