Entry into Cambodia via the international airport in Phnom Penh is really very simple.
On the flight from Ho Chi Minh, I was given all the necessary paperwork, three forms in total.
These were all filled in before I arrived, and I had them in my hand, along with my passport and a passport photo.
Got in the queue, handed across the arrivals form, passport, photo and $20. Was moved along to the next bit, where about two minutes later my passport was returned, the nice guard even called me by my Christian name.
My girlfriend didn’t have a photo. Her wait time was about ten minutes, and cost an extra $4 for the photo (which it must be said, wasn’t seen by either of us). At no stage did I see anyone take a photo of her, there didn’t seem to be any photographic equipment on the desk (unlike on the second desk) but she paid $4 anyway.
If you have your photo, entry into Phnom Penh by plane is very simple indeed.