Phnom Penh is the same as any city and petty crime is something to be mindful of.
Opportunity crime is the most common crime. The most common ways that property is taken are:
A phone loosely held while in a public place is grabbed.
A phone or bag is pulled out of a tuk tuk en route.
Property dragged through a house window using a hook or wire.
Property taken via an open balcony door.
Consider wrist straps for phones, and shoulder straps for bags.
You need to be sensible and use your common sense, be careful of bag snatching and be mindful of your surroundings if you talk on the phone in the street. Common sense strategies that you would use as a visitor to any developing country are necessary.
In your home, put computers, phones or other small valuables away from the window, in a drawer or other safe place out of view/reach.