Cambodia is a quite safe country by world standards. But it is worth noting some precautions. You should use taxis or Tuk Tuk to get around at night, with the aid of a hotel address card to show drivers. They do not cost much but they are much safer than travelling with motorbikes.
Throughout your Cambodia trip, always keep valuable belongings like passport, air tickets in hotel safe whenever possible. Most of hotels have safe in rooms. In Phnom Penh, we recommend you wear inexpensive jewelry while walking on sidewalk or sightseeing.