Please be aware that Bangkok has multiple airports and that these results are for all airports serving the city.
The best month to fly between Bangkok and Phnom Penh is February and the most expensive month to fly is June. (Average prices, based on 297 datapoints.)
The best day to arrive at Bangkok is Thursday and it would be best to avoid Sunday, as prices are on average 11.85% higher than on Thursday. (Average prices, based on 1219 datapoints.)
The cheapest airline flying from Bangkok to Phnom Penh is Thai AirAsia. They are 64% cheaper than Thai Airways. (Average prices, based on 302 datapoints.)