Testing FCM Service...
FCM Service initialized.
Attempting to send notification to dummy token...
Result:
Array
(
    [success] => 
    [error] => Array
        (
            [error] => Array
                (
                    [code] => 400
                    [message] => The registration token is not a valid FCM registration token
                    [status] => INVALID_ARGUMENT
                    [details] => Array
                        (
                            [0] => Array
                                (
                                    [@type] => type.googleapis.com/google.firebase.fcm.v1.FcmError
                                    [errorCode] => INVALID_ARGUMENT
                                )

                        )

                )

        )

    [http_code] => 400
)

Note: An error is expected because the token is dummy. 
If the error code is 404 (UNREGISTERED) or 400 (INVALID_ARGUMENT), authentication worked!
If the error code is 401, authentication failed.