I went directly to PP.. took 3 days to get it (originally they said will be 4 but I literally beg them to hurry the process as I had my ticket booked already to come back).. and am not sure if you have done many 'in and out' or have many stamps in your passport, as they are getting more and more picky
Phom Phen is a no no.. very slow and they are very particular (I just did it last week) and many questions will be raised if you have been in and out frequently