PHP gets a bad rap because of the way many of the people who use it program. The language would be very good, in my opinion, with just a few standardization tweaks to the standard library.

It seems this project has retconned the standard library. I'd say this is a perfect application for this sort of language.

Weak typed, simple to read, simple to write prototyping.

Anyone can pick up PHP, that's its beauty.

Anyone can pickup: Python, Ruby, Lua, whatever. Even the more exotic new ones like Chrystal are very easy to understand. PHP really isnt. A language IMHO cant be simple if you cant derive the method names in the order of the arguments.

But wouldn't it be better to have a language/environment where prototyping is simple, but writing complex apps is also simple?

Writing complex apps in PHP is just fine. You just need to respect the better parts of the language, just like everything else.

I fix PHP apps for a living, and sometimes I write them from scratch if a client is set on using PHP. I strictly follow PSR standards and have tons of mess detector and sniffing rules that keep me mistyping or being less disciplined. I use all the new type hinting features in PHP 7, and before that I was heavily using PhpStorm's excellent support for inferring types from doc blocks.

It's still way, way more painful to write large, complex apps in PHP than in a better-designed language like TypeScript. I actually just rewrote an entire API in TypeScript, and it was like breathing clean air.

I actually realized that having better typing totally eliminates 90% of the tests I was writing for my PHP code. With static typing, you just make sure your inputs conform to the right type, and then you always seem to end up with bug-free code. That's simply not possible with PHP without bending interfaces to suit your needs. You end up with an insane number of unnecessary classes/interfaces instead of simple types that you get in other languages.

It's definitely possible to write complex apps in PHP, especially if you have excellent discipline, but there's a really good reason Facebook created Hack. PHP = Blub