There are definitely no direct buses or trains from Cambodia to Singapore. The closest would be a bus to Bangkok, then bus/train all the way down through Thailand and Malaysia to Singapore. This would be at least 48 hours of travel.
Did you check prices with Tiger Airways and Jetstar? It shouldn't be too hard to find a direct flight from Phnom Penh to Singapore for under $100 USD. Compared to the time, money and effort you'll spend traveling overland, this is very reasonable