It will work in Cambodia or anywhere else in the world. Depending on the bands used by the carriers in Cambodia it may or may not work on LTE, but will always work on 3G.
Two important things to know:
- You MUST buy the phone directly from an Apple Store or Apple Online and explicitly ask for an unlocked phone not associated with any US carrier. No other merchant sells unlocked phones, although some overzealous sales people might imply otherwise.
- The warranty is not international; the phone must be returned to the US for any Apple service, both in or out of warranty.
A less important thing to know: Sales tax will be added to the price, and is not refundable. But it can be as high as 11% depending on the State and City where you purchase the phone. For one datum, New York City sales tax is 8.875%. Across the Hudson River in New Jersey it is 7%.