how to use php://stderr ?(streamHandler, monolog)

I want to use StreamHandler in monolog.

I added the stderr channel to stack channel, and printed out the log.

 'stderr' => [
        'driver' => 'monolog',
        'handler' => StreamHandler::class,
        'formatter' => env('LOG_STDERR_FORMATTER'),
        'with' => [
            'stream' => 'php://stderr',
        ],
    ],

I don't know which directory I can check.

Thank you for reading.