I want to add Bitcoin payments on my website. I've found some PHP library, but as far as I understand it's using only one wallet for incoming payments, and possibly just tracking for confirmations. Is that how all of the payment gateways work? If that's the case, how does it recognize which payment is from whom? For example, if two customers buy the same thing on the same time, and only one pays for it, how would it know which one paid? Thanks!