Guzzle is a PHP HTTP client that makes it easy to send HTTP requests and trivial to integrate with web services. We are using Guzzle to integrate PayPal API.
You can use the PayPal Payments API, documented at: https://developer.paypal.com/docs/integration/direct/rest-payments-overview/ to let your users make payments to you of dynamically/arbitrarily generated sums.
To use the API, first you would need to get your PayPal Client and Secret Id, which are unique strings given to you in exchange for creating an application. You can create an application at: https://developer.paypal.com/developer/applications/create