Well, first of all, you need a contract with an online bank (for actual transactions).. Well, you obviously know that.. They will probably provide
you with a connection to some page on their site where you will connect and commit transactions.
Anyway, the easiest way to accomplish this is to install PHP with cURL library, which u can use to connect to their web-page within your script, post the transactions, get the result and use the result in your pages.
Since this is a very wast topic, you should first contact some bank and ask them about options of connecting to their online system. After that, you can ask more specific questions.
Regards, alex.
Alex: My favorite site is
http://localhost/
R.J. Oppenheimer: "I am become death, destroyer of worlds" (1945 AD)