How to implement Real-time notifications (Pub/Sub) for in-App-Purchases?

I am trying to update my database according to the push notifications that come from Google when a user purchases or subscribes in my app.For this I will use php. What will happen is:

1. User makes a subscription in my app.
2. Pub/Sub receives that data and sends a push notification to a link provided by me.
3. Data in database is updated. 4.The app makes an http request and gets the subscription information.

I provided an URL for pub sub notifications , what I need is to parse the data, make a request to get the updated data and update the database accordingly . I know how to make the request

    $client = new \Google_Client();
    $client->setAuthConfig('credentials.json');
    $client->addScope('https://www.googleapis.com/auth/androidpublisher');
    $service = new \Google_Service_AndroidPublisher($client);
     $subscription = $service->purchases_subscriptions->get($package_name,
            $product_id,
            $token);

But I don't know how to handle subscription or purchase coming from PubSub and flow accordingly. How should I do it?