Query Help

[TNChalise]

I have a real-time database with the following data structure.

From the javascript I can query like this:

 const conv = await database
    .ref()
    .child('conversations')
    .orderByChild(`users/28`)
    .equalTo(true)
    .once('value');

Which is giving the required results.

I am trying to do the same on the PHP side. I am using laravel version of this same library.

$database      = app('firebase.database');
$conversations = $database->getReference('conversations')
                   ->orderByChild('users/28')->equalTo(true)->getValue();

But this throws an error:

{ "message": "Index not defined, add ".indexOn": "users/28", for path "/conversations", to the rules"}

I have the following rule configured:

How can I query to filter conversations that user 28 is participating? PHP Side.