Originally Posted by goodgirl

Thanks Kaspars, that's the best case. No cash needed when using Grab with saved credit or debit card.

In Cambodia an app called "PassApp" seems to be used for rides as well. To sign up you need a Cambodian number, which is easily available because local SIM cards are extremely cheap, either for free or for just a few USD at the airport or mobile phone carriers shops like "Smart".
Here is an overview of the networks / SIM card options in Cambodia: https://prepaid-data-sim-card.fandom.com/wiki/Cambodia