One of the cheapest place to eat that I found in Phnom Penh is located at Street 258. This location is near budget guesthouses such as Kha Vi Guesthouse and the popular hostel for backpackers, Okay Guesthouse. This local restaurant is just beside The Rouge Shop and Gecko Guesthouse.
They have a variety of cheap foods that are freshly prepared when you order and some cooked Cambodian dishes as well. What I usually order is fried rice with egg and pork (also available with pork and chicken).
For $1.50, it’s a great deal already.
If you’re looking for a cheap place to have lunch in Phnom Penh, check this one out.