Hey everyone,
There was a brief discussion on this list on the subject of Composer use in MediaWiki last October [1]. I also answered various questions regarding Composer in a tech chat via Google Hangout in November. I never responded to the mailing list thread and thought I'd bring up a few points regarding the statements made then.
Composer is for the most part a library handling package metadata, package repository metadata, version matching, dependency resolution, downloading and unpacking packages as well as the processes that put all of these parts together in an extensible way.
Composer originated from the ideas of a new plugin system for phpBB as well as an installer for Symfony2 bundles, which we realized was really a generic problem, so Composer has no direct ties to either of these. However it was designed with these use cases in mind. More specifically that means that while Composer comes with a nice default command line interface, it is also very well suited for creating project-specific package management tools on top of it, including web based ones if that is of any interest.
There are multiple mechanisms in Composer which allow customization while keeping the standard command line interface. Most notably custom installers [2] which allow packages of a particular type to be installed using custom installation code which allows one to modify pretty much everything, and scripts [3] which provide ways of triggering either PHP code or external programs based on various installation/update/removal events.
If this alone is not enough for MediaWiki, you can simply use Composer as a library saving you a lot of work when building a custom extension management tool.
I'm happy to answer further questions, and you'll reach plenty of people with composer experience on #composer and #composer-dev on FreeNode, too.
Cheers, Nils
[1] http://lists.wikimedia.org/pipermail/wikitech-l/2012-October/thread.html#640... [2] http://getcomposer.org/doc/articles/custom-installers.md [3] http://getcomposer.org/doc/articles/scripts.md