Western style or very upscale local restaurants and bars
International and major local hotel chains
Some airport shops
That's about it. You'll be hard pressed to find card acceptance outside of these types of establishments in Phnom Penh.
Outside of Phnom Penh, with minor exceptions you should assume that no one will accept credit card
Also, cards like American Express is basically not accepted anywhere
Most local businesses transact in cash, which is what you should rely on. But as I said here, you can pay for most things with USD directly without converting to the local currency.