The thing is you have to be a medium-to-expert to be confident about Composer dependencies, every new dev is going to use the language API, which resembles a bunch of functions with no general pattern.

PHP users need to keep it together and set a common base, a standard library with sane defaults.

I am new to PHP and I started right off with composer -- all you have to do is composer require pkg dev-master [or other pin].

I don't see how this is different from gem install pkg.

Some of the biggest names in PHP are working on that exact idea!

With previous versions very stable, testable and modern applications could be built as well.

Things like object type hinting can help building stable apps but imho it is still the work of the programmer that can break or make it.