First payment gateway? [closed]

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I'm recently started a project where the client requested I add a payment gateway to it. My first thought was fear, I've always avoided working with cash (because I've never trusted my code that much, and don't pretend I know enough about sec开发者_如何学JAVAurity), but the more I thought about it, the more I realized it shouldn't be that hard to implement.

So my question is: How hard is it to implement this? Should I hire an outside party to look over my code, etc. What steps should I take to make sure the gateway doesn't get attacked?

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If at all possible use PayPal. They have extensive documentation and it is relatively simple to implement. As an added benefit you are able to rely on them for payment security. And they have a very good reputation. I have only ever used paypabut there are other systems similar if you can't for some reason.