The government wants to launch a campaign to facilitate access for people with reduced mobility, as much for tourists as for the local people.
In a country where many of the citizens have suffered from war and anti-personnel mines, the lack of infrastructure and help for disabled people is striking. Lots of restaurants, and even hotel complexes, are not equipped to welcome guests with reduced mobility. Some hotels don't even have an elevator!
Fortunately, more and more hotels are adapting themselves: the large hotel chains that are in Phnom Penh and Siem Reap are equipped with all the comfort and service necessary. Even if you have a smaller budget, lots of hotels in the city centre have disabled ramps and the staff are always more than happy to help with anything you might need.
As far as visits to the temples are concerned, it is possible to organise day trips and get there by car or tuk tuk. However, some temples require you to walk up lots of steps, such as the Angkor Wat temple.
You will realise during your stay in Cambodia that there is a distinct lack of pavements in the country. They are there in the large cities but are mainly used for small shops or motorbikes. Therefore they are inaccessible. So, for those with reduced mobility, you should prioritise travelling by car or mini-van to ensure comfort and security.
There is also no public transport in Cambodia, so you must make sure beforehand that you hire suitable private transport to ensure a comfortable stay.