Credit card processing has two steps: authorization and capture. The first step is when you make sure that the transaction is authorized, which involves checking validity, balance etc and the second step is the actual capture of the funds from the client's credit card issuing bank to the merchant bank. We have successfully used Cybercash's software for this (http://www.cybercash.com
). They give you all the SDK's for building prototypes and also the necessary licenses for actually implementing this in your site as well. They provide the necessary APIs which you need to call from within your application for performing the Credit card validation.