I recently came across the Phive project and have had a play with it. Phive is part of phar.io and is intended to manage development tools such as PHPUnit in preference to using Composer's dev dependencies. The main advantages of Phive are that it uses the phar file of the tool and only keeps one copy of each version rather than downloading a new copy into each project.
He starts off talking briefly about how the tool works and what it's doing behind the scenes to download the phar archives requested. He shows how to define a target directory, install for global use and explains how GPG signatures work into the installation process.