I have worked on OpenCart and Magento, both being PHP MVC Framework meant for building eCommerce Websites while I have also worked on Wordpress which is a CMS PHP Framework which has several woo-commerce plugins. All the mentioned PHP Frameworks have many free and paid themes for eCommerce aesthetics.
While I have worked on core PHP and the other PHP MVC Frameworks like Laravel, CI, YII, CakePHP, etc to build all sorts of websites out of which there were few eCommerce website as well.
I have worked on many Payment Gateway / Providers as well, for example, Braintree, Stripe, Paypal, Authorize, Skrill, AlgoCharge, Neteller, etc.
I have integrated and implemented many other APIs too which would be a long list to mention.