Composer

composer is a dependency manager for PHP libraries. In addition, it can be used to manage the installation of MediaWiki extensions (as of MediaWiki 1.22). Starting with 1.25, MediaWiki core's external dependencies are being managed with composer.

Installing Composer
Download Composer. See the instructions on that download page for all options. Important to note is that one can simply get the composer.phar file and run Composer via this file.

wget https://getcomposer.org/composer.phar php composer.phar someCommand

This is equivalent to doing an actual installation of Composer and then running

composer someCommand

Resources

 * How to install extensions through Composer
 * Extension:Extension Installer - must be used for MediaWiki ≤ 1.21.x to install extension with Composer
 * List of extensions that can or must be installed with Composer
 * Category:Extensions supporting Composer
 * How to add external libraries to MediaWiki core or extensions
 * Best practices when creating a composer.json
 * Information about adding packages to packagist.org