Steps to reproduce:

1. composer create-project symfony/skeleton my-project
2. cd my-project
3. composer require --dev profiler webserver
4. bin/console server:start
5. Navigate to http://127.0.0.1:8000/foo/bar
6. You get a 404 as expected and the profiler toolbar shows the dev environment as expected
7. mkdir public/foo
8. Again navigate to http://127.0.0.1:8000/foo/bar
9. Notice the profiler toolbar is missing and if you inspect the $env var in public/index.php it is an empty string

I think this bug is related to #23799 (https://github.com/symfony/symfony/pull/23799/files#diff-17a9fecf57df3ecbd28ddf129c45ff70R157) but I'm not sure why creating a the /foo directory causes this issue.

A work around is to change the following in public/index.php:

if (!isset($_SERVER['APP_ENV'])) {

to:

if (!isset($_SERVER['APP_ENV']) || !$_SERVER['APP_ENV']) {