By International Flights
Cambodia has two main international airport; Phnom Penh & Siem Reap. If you catch international flights from Europe countries, there will be 1 or from 2 transits depending on the country your flight is from and the last transit will be in Bangkok or in Ho Chi Minh. The direct international flights to Cambodia are generally from Asian countries such as Laos, Vietnam, Thailand, Philippine, Malaysia, Singapore, Hong Kong, Korea.
Border Crossing / By Buses or Mini Van
If you enter from Thailand, Laos & Vietnam through the border crossing, here are the popular borders that most tourists generally across to Cambodia. Public bus tickets can be reserved from travel agencies or from the hotels you’re staying. Tourists mainly take public buses to Cambodia either from Ho Chi Minh or Bangkok by Giant Ibis buses while there are very rare public buses from Don Khon (4000 islands), Laos to Siem Reap or Phnom Penh but there are some mini vans (15 seats) run daily from Don Khon to Cambodia.
The alternatives are private taxis running daily direct from the borders (Thailand, Vietnam, Laos) to Siem Reap and Phnom Penh. You have to be careful about the scams or cheating (currency exchange) at Poi Pet (Aranyaprathet) border as there have been these issues happened to tourists.
› Thailand: Poipet (Aranyaprathet) to Siem Reap by overland takes approximately 2 hours and half. Koh Kong border crossing (Had Lek) is often crossed by those who travel by overland from Phnom Penh and spend a few nights for beach break in Kep, Sihanoukville or itself Koh Kong then continue to visit Thailand. Other border crossings: O’Smach, Chong Sa Ngam, Duan Lem, Pailin.
› Vietnam: Ba Vet (Moc Bai) & Prek Chak (Ha Tien) [or Vice Versa] are the most popular border crossings after visiting Vietnam, then by overland to Phnom Penh from Ba Vet which takes around 4 hours or spend several nights for beach leisure in Kep/Kampot if they cross the border from Prek Chak (3 hours from Kep) before continuing to Phnom Penh. Other border crossings: Trapheang Phlong, and Phnom Den.
› Laos: Dom Kralor (Veun Kham) is the international border crossing from Laos to Cambodia and it takes about 8 hours by overland to Phnom Penh and 6 hours to Siem Reap.
Border Crossing / By Speed Boat
› Vietnam: There another border crossing is by boat (6 hours), called “Kao Orm Samnor” if you visit and cross from Chau Doc, Vietnam.
Learn more: https://www.triplekangkor.com/indochina-travel-guides/getting-to-cambodia/