Drugs, including cannabis, are illegal in Cambodia, and penalties can be very severe. That said, enforcement tends to be on the lax side and many guesthouses are permanently shrouded in purple haze. Low-grade cannabis (ganchaa) is fairly common in Cambodian cooking (for the flavor), but the days when you could just walk up to the Central Market and buy a kilo are over.
Both Phnom Penh and Siem Reap are full of Happy Herb pizzerias, but the police crack down occasionally, so even if you ask for "extra happy" and try out your secret handshake, you may only end up with lawn clippings. Alternatively, if they do deliver, be warned that effect of eating Happy Pizza comes on only slowly and you may end off biting more than you can chew, so proceed with caution.
Heroin is very high grade in SE Asia and foreigners requesting cocaine are sometimes provided with it instead, this regularly leads to deaths.