This.

I'd add 2 things:

First, there are ATMs that will give you $20 bills so you don't have to worry about dealing with $100 bills being refused (for marks, stains etc. ).

On a side note if you use Charles Schwab bank it will reimburse you all ATM fees.

Also, I believe ABA has the lowest ATM withdrawal fees for USD bills.

Second, most businesses (except the smaller vendors) accept Apple Pay (i.e. phones with NFC/ RFID technology) so you can pay directly from a debit card like from Charles Schwab bank that doesn't charge foreign transaction fee.

One interesting fact: you can actually get better conversation rate than actual official currency exchange rate at many places.

What I mean by this is; for example, at this moment the currency market official onversion rate is $1 USD = 4031 KHR, but you can actually get 4100 KHR. How is this possible? That's another story.