Contents
Intro
Composer is one of many Dependency managers. Composer makes it easier to distribute PHP code that relies on other code.
in MediaWiki
mw:Composer is used in the MediaWiki engine to manage all core libraries and dependencies since version 1.25
Installation
Installing it on your own host would look something like this
curl -sS https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/composer
echo 'export PATH="$HOME/.composer/vendor/bin:$PATH"' >> ~/.bashrc
source ~/.bashrc
# now you can use it to do things like install the latest version of Drush
composer global require drush/drush:dev-master
while installing it on a shared host like WebFaction would be slightly modified
ln -s `which php54` ~/bin/php
curl -sS https://getcomposer.org/installer | php54
mv composer.phar ~/bin/composer
echo -e '\n# Composer\nalias composer="php54 $HOME/composer.phar"' >> ~/.bash_profile
source ~/.bash_profile
which composer
# now you can use it to do things like install the latest version of Drush
composer global require drush/drush:dev-master
drush --version
More
To use Composer in your PHP project, see this primer, and also this handy-dandy cheatsheet (with mouseovers!)