Caddyfile

0.0.0.0
root ./
gzip
fastcgi / 127.0.0.1:9000 php {
    env FOO bar
}
rewrite {
    regexp .*
    ext /
    to /index.php?{query}
}

log stdout
errors stdout

index.php

<?php

// You have to uncomment the following line if the ini directive auto_globals_jit is On.  Otherwise the HTTP env vars will be missing.
// var_dump($_ENV['ANYTHING'] ?? false);
var_dump(getenv());
var_dump(getenv('FOO'));

output.txt

» http localhost:2015
HTTP/1.1 200 OK
Content-Encoding: gzip
Content-Length: 590
Content-Type: text/html; charset=UTF-8
Date: Fri, 22 Mar 2019 19:21:22 GMT
Server: Caddy
Vary: Accept-Encoding
X-Powered-By: PHP/7.1.25

array(34) {
  ["USER"]=>
  string(4) "matt"
  ["HOME"]=>
  string(11) "/Users/matt"
  ["GATEWAY_INTERFACE"]=>
  string(7) "CGI/1.1"
  ["AUTH_TYPE"]=>
  string(0) ""
  ["HTTP_USER_AGENT"]=>
  string(12) "HTTPie/0.9.9"
  ["FOO"]=>
  string(3) "bar"
  ["DOCUMENT_URI"]=>
  string(10) "/index.php"
  ["SERVER_PORT"]=>
  string(4) "2015"
  ["CONTENT_TYPE"]=>
  string(0) ""
  ["HTTP_ACCEPT_ENCODING"]=>
  string(13) "gzip, deflate"
  ["REQUEST_URI"]=>
  string(1) "/"
  ["HTTP_CONNECTION"]=>
  string(10) "keep-alive"
  ["QUERY_STRING"]=>
  string(0) ""
  ["REMOTE_HOST"]=>
  string(3) "::1"
  ["PATH_INFO"]=>
  string(0) ""
  ["HTTP_ACCEPT"]=>
  string(3) "*/*"
  ["HTTP_HOST"]=>
  string(14) "localhost:2015"
  ["SERVER_NAME"]=>
  string(7) "0.0.0.0"
  ["REMOTE_PORT"]=>
  string(5) "53670"
  ["SERVER_PROTOCOL"]=>
  string(8) "HTTP/1.1"
  ["DOCUMENT_ROOT"]=>
  string(23) "/Users/matt/code/env-ex"
  ["CONTENT_LENGTH"]=>
  string(1) "0"
  ["SCRIPT_NAME"]=>
  string(10) "/index.php"
  ["REMOTE_USER"]=>
  string(0) ""
  ["REMOTE_ADDR"]=>
  string(3) "::1"
  ["SCRIPT_FILENAME"]=>
  string(33) "/Users/matt/code/env-ex/index.php"
  ["REQUEST_METHOD"]=>
  string(3) "GET"
  ["REMOTE_IDENT"]=>
  string(0) ""
  ["SERVER_SOFTWARE"]=>
  string(12) "Caddy/0.11.5"
  ["REQUEST_SCHEME"]=>
  string(4) "http"
  ["FCGI_ROLE"]=>
  string(9) "RESPONDER"
  ["PHP_SELF"]=>
  string(10) "/index.php"
  ["REQUEST_TIME_FLOAT"]=>
  float(1553282482.55)
  ["REQUEST_TIME"]=>
  int(1553282482)
}
bool(false)