Most visitors to Cambodia require a one-month tourist visa $30, although some visitors enter on a one-month business/ordinary visa $35. Most nationalities receive a one-month tourist visa on arrival at Phnom Penh and Siem Reap airports and at land borders. One passport-sized photo is required and you’ll be charged $2 if you don’t have one. ASEAN nationals get one month free entry. It is also possible to arrange a visa through Cambodian embassies overseas or an online e-visa (USD30 + USD7 processing charge) through the Ministry of Foreign Affairs (www.evisa.gov.kh). Arranging a visa ahead of time can help prevent potential overcharging at some land crossings.

Those seeking work in Cambodia should opt for the business visa as it is easily extended for long periods and can be extended to include multiple entries and exits. If you plan to work in Cambodia you will need a work permit too. A tourist visa can be extended only once for one month, and does not allow for re-entry.

Travelers are sometimes overcharged when crossing at land borders particularly with Thailand as immigration officials demand payment in baht and round up the figure considerably. Arranging a visa in advance avoids this potential problem. Also, the touts will tell you that you need to exchange foreign currency for Cambodian riel. THERE IS NO REQUIREMENT TO CHANGE MONEY AT ANY BORDER. It is a scam to get you to change money at a rip-off exchange rate. Particularly do not change money into Cambodian riel, we all use US$, riel is for small change only (less than 1US$).
Travelers planning a day trip to Prasat Preah Vihear from Thailand do not require visas but may be asked to leave their passport on the Thai side of the border to ¬ensure they don’t continue on into Cambodia.

Overstaying your visa currently costs $5 a day.

Visa extensions
Visa extensions are issued by the large immigration office located directly across the road from Phnom Penh International Airport.

There are two ways of getting an extension (one official and one unofficial) and unsurprisingly, the time and money involved differ greatly. Officially, a one-month extension costs US$35, three months US$65, six months US$125, and one year US$200; your passport will be held for 25 days and there will be more paperwork than a communist bureaucrat could dream up. This is fine for expats with an employer to make the ¬arrangements, but those on their own really need to go unofficial. Using an agent, you can have your passport back the next day for roughly US$50 for one month, US$90 for three months, US$200 for six months and US$300 for one year. Travel agencies and some motorbike rental shops in Phnom Penh can help with arrangements.

E-Visa: https://www.evisa.gov.kh