How does Ratchet (socketo.me) broadcast to users subricbed to a certain topic?

Question

I have been following the directions on the website for socketo.me ratchet, Which has been perfectly fine so far. I am interested in broadcasting topics to users that only subscribed to it, and not everyone.

This code given below, and also here on their website is confusing me.

<?php
use Ratchet\ConnectionInterface as Conn;

/**
 * When a user publishes to a topic all clients who have subscribed
 * to that topic will receive the message/event from the publisher
 */
class BasicPubSub implements Ratchet\Wamp\WampServerInterface {
    public function onPublish(Conn $conn, $topic, $event, array $exclude, array $eligible) {
        $topic->broadcast($event);
    }

    public function onCall(Conn $conn, $id, $topic, array $params) {
        $conn->callError($id, $topic, 'RPC not supported on this demo');
    }

    // No need to anything, since WampServer adds and removes subscribers to Topics automatically
    public function onSubscribe(Conn $conn, $topic) {}
    public function onUnSubscribe(Conn $conn, $topic) {}

    public function onOpen(Conn $conn) {}
    public function onClose(Conn $conn) {}
    public function onError(Conn $conn, \Exception $e) {}
}

This information below on their website I am interpreting that it should be integrated with public function onPublish.

Events triggered by this Component:

onPublish (ConnectionInterface $conn, Topic $topic, string $event) - The user publishes data to a $topic. You should in return an Event Command to Connections who have Subscribed to the $topic

WAMP:

(string $sessionId) - A unique ID given to the client

(array $subscriptions) - A collection of Topics the client has subscribed to

Shouldn't their be some code like along the lines of this

class BasicPubSub implements Ratchet\Wamp\WampServerInterface {
    public function onPublish(Conn $conn, $topic, $event, $sessionid, array $subscriptions, array $exclude, array $eligible) {
    $topic->broadcast($subscriptions);
    }
/** rest of code here */
}

Clearly $subscriptions will be populated by an sql query to retrieve the topics a user has subscribed to based on thier $sessionid, and $topic->broadcast($subscriptions); will broadcast the topic to individual users subscribed. I do not see where the tutorial is doing this, clearly my interpretation is different. I need Help!!!