International departure tax is $20 at Pochentong, $15 at Siem Reap. Domestic departure tax is $5. The national carrier is not operating at this time, so three private airlines handle the domestic flights, which are safe enough that you don't have to worry about it.
Siem Reap Airways, which is part of Bangkok Airways, flies French-built ATRs between Siem Reap and Phnom Penh a couple of times a day. Royal Phnom Penh Airways flies Chinese built Y7s between Phnom Penh and Siem Reap/Battambang and Phnom Penh and Stung Treng/Banlung. President Airlines runs the same routes as Royal Phnom Penh using Russian-made Antonov 24s.