Payment Processing - Credit Cards - Any Libraries available?

I have developed for both authorize.net and paypal with Codeigniter. Both were fairly easy to integrate. If I recall correctly authorize.net has a nice SDK and PHP examples that adapted to CI nicely. That site did require PCI compliance and it was a pain.

The Paypal experience revolves around the use of "Buy it Now" buttons. Which was not hard to integrate. No PCI work needed as the system redirects to PayPal where all the card and other sensitive info is handled. Reasonably easy to use their payment notification to keep the website backend "informed".

Paypal offers several other APIs beyond the buttons. Have spent little time exploring those so - no comment.

You need SSL for Paypal these days.

There have not been any problems with Paypal account-freezing shenanigans that I have read much about. That (knock on wood) seems to be a resolved issue AFAIK. Authorize on the other hand was a royal pain. Money moved about easily but they had a tendency to change the terms of use without warning and the same with rates, fees and "other" charges. It got very expensive for one very small retailer. Paypal has worked much better for them.