Money: The riel is the official currency of Cambodia, but US dollars are more commonly used by businesses, even in the markets. Riel is generally used for only small purchases. ATMs accept most bankcards, including Union Pay. Check your change, though, as stores and restaurants will not accept notes that are even slightly ripped, nor can you use $2 notes.
Visas: Landing visas for tourists cost $30 per person at Siem Reap International Airport. Try to fill out the paperwork on the plane there to save time, and if possible pay with the exact money.
Scams: Inside temples, be wary of staff members demanding an extra fee to enter “closed-off” sections; more often than not, they are the ones who closed off that section. Also, the people casually offering incense near statues will expect money if you take it off them.
NGOs, orphanages: There are many ways in which tourists can contribute to aid organizations, such as eating at Haven restaurant, which supports vulnerable young adults, or donating blood at the Angkor Hospital for Children. However, carefully research any orphanage you plan to visit, as some are scams using children to profit from the so-called pity industry.
(China Daily 04/25/2015 page17)