I am running an Expression Engine site with the shop module Expresso Store. For that module i need a new off-site payment gateway similar to paypal express. The payment gateways in Expresso Store are based on the Omnipay Payment Processing Library: [url removed, login to view]
Have a look there how the existing payment gateways are written.
About the new payment gateway:
Making a payment is quite simple (that works already): it's just a form with a few hidden input fields (price, currency, etc.) and a hidden sha1 key. After the client hits the submit button he is redirected to the payment institute where he gives in all the sensitive data like credit card number etc. After finishing the payment process at the extern site the payment institute redirects him to my site again and adds all the transaction details as parameters in the url (transaction status, amount paid, sha1, etc.).
At this point the new payment module has to fetch the url parameters and update the Expresso Store Database.
Detailed information about the payment institute you get after bidding..