Omnipay is a payment processing library for PHP. It has been designed based on ideas from Active Merchant, plus experience implementing dozens of gateways for CI Merchant. It has a clear and consistent API, is fully unit tested, and even comes with an example application to get you started.
Coming from The League of Extraordinary Packages, it seems to be a more popular solution than Payum and the others. It also looks like Omnipay supports way more gateways than any other payment processing library that I’ve seen. Here’s the list of the officially supported gateways. Here’s the list of the third-party contributed gateways. And, of course, you can build your own.