Get the payment Object by passing paymentId payment id was previously stored in session in CreatePaymentUsingPayPal.php
PaymentExecution object includes information necessary to execute a PayPal account payment. The payer_id is added to the request query parameters when the user is redirected from paypal back to your site
(See bootstrap.php for more on ApiContext)
Execute Payment Sample
This sample shows how you can complete a payment that has been approved by the buyer by logging into paypal site. You can optionally update transaction information by passing in one or more transactions. API used: POST '/v1/payments/payment//execute'.