The answer is that Thai Airways, no surprise, flies from Cambodia to Bangkok and from Bangkok to Dubai. The flight possiblities on TG from PNH to BKK are:
TG697 1005 1110
TG699 2025 2130
However, you should know that there are twice as many flights on the combination of Bangkok Airways or ThaiAirAsia.
From Bangkok to Dubai, there are:
TG4503 0125 0430
TG517 1800 2130
and again twice as many flights on Emirates
I checked out whether TG flies from either HoChiMinh City or Hanoi to Phnom Penh, but it does not. I also checked whether it flies from Japan to anywhere, but it does not (not even as part of combinations).