回复：谢谢freebirdie 提供的信息, 记下了. 还有个请求.......

I applied evisa since my mom holds Chinese passport, unless you have the third county's visa, the Chinese immigration won't let Chinese citizen exit.  If you don't need to worry about this,  you can just go and do the airport visa.

The evisa's link is here , very easy and fast.

http://www.mfaic.gov.kh/evisa/

BTW, if you stick to the temples, it's safe as there are so many tourists.