Visa extension can be done only in Phnom Penh. I directly went to the immigration department which is in front of Airport. It is very easy to find the place.
I got my visit visa extended to one month. The official fee is 30 USD for one month. A passport size photo is needed. It is easy to fill the application form. You should have the address of your hotel or apartment. The officers are friendly and straight forward. Passport can be collected in 5 working days.
If you are from a developing country, it is better for you to go yourself because you can answer any question. To go to airport from city a moto rider takes about 4-5 dollars. So I had to spend about 48 USD altogether. Now an agency takes 50 USD to give the service.