It looks like starting with version 3.3.x in the symfony/process component, there's an issue with inheriting environment variables when $_ENV is empty - which is the recommended setup for production systems (see http://php.net/manual/en/reserved.variables.environment.php#98113).

Could be solved by including an extra loop with getenv() in Process::getDefaultEnv(), but not sure if there's a rationale to not do this.

This bug might cause hard to find bugs - i.e. I discovered this issue after debugging an issue for hours where a headless chrome instance wouldn't listen to the specified port when ran with --remote-debugging-port=9222, turned out it was missing some - apparently - critical env vars.

Reproduce:

$_ENV = [];
$process = new PhpProcess(<<<EOF
<?php var_dump(getenv()); ?>
EOF
);
$process->setEnv(getenv()); // <-- comment out this line to see bug
$process->run();

echo "<pre>".$process->getOutput();

Edit: seems to conflict with #25559, though I think that could be solved by introducing a loop and filtering out all non-string values, like in the other loops.