Most tools out there allow you to shoot yourself in the face. In this case, the guy replaced client-side JavaScript (very notorious for its amazing face shooting abilities) with server-side PHP (also, a very notorious face shooter).
The real metric of quality is does it do what you expected. PHP is a gun that sometimes shoots backwards with no sensible reason where as rust has a gyroscope that can work out when the gun is pointing at your face and requires that you turn off the safety first.
Entire classes of bugs simply don't exist in better languages.
Well, yes. You can write a web application in Haskell and enjoy the beautiful and flexible type system, first order functions, monads and all the other wonderful things. But you shouldn't. Because when you leave, the client will never be able to find someone to maintain the damn thing and chances are they will hire someone else to rewrite it yet again in PHP. And possibly hire someone to make a voodoo doll of you and stick pins up its ass.
