@Nathan *******
, well what ever countries passport you used to ENTER Cambodia (if you did it by LAND) that's the passport you'd use to EXIT Cambodia and enter thailand
BUT most borders don't let (Argentina, Brazil, Chile, Peru OR South Korea bounce because those people get 90 day visa exempt entries NOT 30 days
GO to the Phnom Penh airport and FLY in to thailand AND do it BEFORE midnight tonight when the new arrival requirements take effect