Simple applications like crud apps don't need unit tests . except when you have very custom features and class (this if you use good tested framework like laravel in php or ror in ruby)
— Depends entirely on where they're used, how stable they need to be, etc
— Like you said it depends
— And if you want a good app, you'll of course avoid PHP at ALL costs
— Did you used this onion arch with js before in real project?