Khmer Smart Keyboard is the best khmer keyboard for the iOS8.x and you can find it on the AppStore for free.
Its way of usage is very simple and useful with these following :
. To delete a character you just simply swipe to the left on your keyboard
. To change keyboard layout just swipe to the right on your keyboard.
. Swipe down on a key to choose the lower one character on the keypad
. Swipe up on a key to choose another character on the other keyboard layout
Note : Don't forget to swipe left, right, up and down on a key, it makes differences.!!!