When you receive paypal payments through your app, you will receive an order in Mobidonia Studio.
But sometimes you need to be instantly notified about the payment.
That's why PayPal has IPN (Instant Payment Notification). Paypal sends complete info for the payment on the specified url.
We recommend the PHP code sample as the most complete one and the most easier to start with. In this example all you have to do is to uncomment the lines from 113 to 121 and use them. For example you can send automatic email using some of the data you have received from paypal.
You can test your code using the IPN simulator her
Also make sure you have enabled IPN URL in you paypal app.
Read more here.