You can apply for Chinese visa there in Cambodia if you hold work permit or residence permit or valid visa in Cambodia. Things are getting difficult for foreigners applying China visa at a third country, you should have valid visas or valid Residence Permits in Cambodia.
One entry tourist visa is US$30. You need a application form, a passport valid for at least 6 months and a passport photo. The General processing time is 4 working days (they visa office only accept application from 08:00 to 11:30 in the morning), express and rush service are available but you need to pay extra fee.

hope i am replying in the right box. so just to confirm if i have a 30 day tourist visa for Cambodia can i apply for i Chinese visa?
thanks

Ms.bling, I can't give you an assuring answer, sometimes, the valid visa to Cambodia is accepted, however, sometimes, it is not accepted because it can't prove you live you in Cambodia for more than 6 months, I can only say that please have a try. It is up to the new vsia regulations and the consular officer.