Visas for Cambodia are required by everyone other than nationals of Laos, Malaysia, the Philippines, Singapore, Vietnam, Thailand and Indonesia. Visas are issued on arrival for $20 at Phnom Penh and Siem Reap international airports, at Sihanoukville port, at all overland crossings from Thailand and Vietnam, and at Voen Kham from Laos.
A single-entry tourist visas obtained on arrival ($20 one passport photograph required, or pay a small surcharge – usually a dollar or two – to have your passport photo scanned) is valid for thirty days, including the day of issue, and can be extended once only, for one month. Note that at the Thai border Cambodian officials may ask for an bit more than the official fee – having an e-visa avoids this hassle. You can also buy a business visa ($25; one passport photo) on arrival. Like the tourist visa this is valid for thirty days, but can be extended in a variety of ways (ranging from one-month single-entry extension, three months’ single-entry, six months’ multiple-entry and twelve months’ multiple-entry; costs range from $42 to $270). Multiple entries are only available on a business visa.
Single-entry, thirty-day tourist e-visas are available online at e-visa.mfaic.gov.kh for $20 plus a $5 processing charge, although they are only supported if you enter through the airports at Phnom Penh or Siem Reap, or overland at Koh Kong, Bavet and Poipet. They must be used within three months of the date of issue. They’re mainly useful if you’re entering via Poipet and wish to avoid the traditional hassles associated with that crossing.