In Phnom Penh, violent crime towards foreigners is rare, but petty crime is unfortunately common and can happen anywhere. The biggest threat is bag-snatching or pickpocketing, so some simple safety precautions are necessary to avoid being a target. Carry a bare minimum of valuables and keep them in a concealed money belt. When taking a tuk-tuk or motorcycle taxi, don’t leave bags where they could be snatched by a passing motorcyclist. If renting a motorcycle, keep valuables under the seat if possible. Avoid walking around alone in areas where there aren’t many people, especially after dark.
Lastly there have been incidents of solo female travellers being sexually assaulted while taking the night bus. If taking the night bus, insist on sitting near the front if the bus isn’t full, and buddy up with another traveller if possible. The safest option, of course, is to avoid the night bus altogether.