PHP中的双向加密——需要一些指导

I'm working on my first secure shopping site. We're not storing credit card data, so that's not a problem. However, we do have a transaction key and API login key for our payment gateway (authorize.net) that I'd prefer to keep in a database, rather than hard-coding into my php. I don't know that we need tremendous security, but I would rather not store it in plain text. I know about sha, but that's one-way. I need a way to store the value in the database in a semi-secure format, but then be able to "decrypt" it programmatically for use in my function. I'm working on my first secure shopping site. W