"When was the last time you looked at a PHP project and saw a list of all the third-party libraries and respective version numbers being used written in a single file?"
Huh? Any serious, modern PHP project uses Composer (http://getcomposer.org/).
Huh? Any serious, modern PHP project uses Composer (http://getcomposer.org/).
Have a look at http://packagist.org
That it's labelled alpha by the developers?
https://github.com/composer/composer/releases
That it's barely 2 years old?
I'm not sure what pointing at packagist proves. There's a lot of packages published for it. Great. THE most popular package installed via packagist's counters is Doctrine, at 1.4 million installs. It's a lot, but compared to all the PHP installs out there, and all the PHP apps out there that aren't using composer (because it's alpha and not very old yet, my conjecture) dwarf the 1.4 million installs of a program via packagist. I'm probably 20 of those Doctrine installs myself.
I was referring to the statement "Any serious, modern PHP project uses Composer". There are loads of 'serious' PHP apps that don't use it, or may only be adopting it now. If the definition of 'modern' means "has to use composer", then, of course, "serious modern PHP projects" use composer.
I personally know people who aren't allowed to rely on it yet in their companies because it's still labelled 'alpha'.