generally,Photoshop has the problem that you see khmer unicode is not working.
You can fix the problem by going to change in the application options.
Try to follow instruction as below.
Open Photoshop and go to Edit Menu >Preference >Type and make sure you choose text engine options Wold-Ready Layout (for photoshop 2020) or photoshop older version you just Select Middle Eastem and South Asian. 
next,you must check in Window > Character and Window > paragraph.
- in Window > paragraph ,make sure you choose and tick on Worl Ready Layout and Middle Eastern Features.
- in Window > Character ,make sure you choose khmer language
after you config with that please check Type > language options > Middle Eastern Features (you will see tick icon here)
Then restart the application and check does it work or not.
the result:
