On the Fortrabbit blog there's a screencast posted showing you how to use Composer using its "create-project" functionality.

Learn here about one way (out of many) to install the famous Symfony2 (a popular PHP framework to build web projects. Also the base for some CMS systems, like the new Drupal8) on the fabulous fortrabbit PHP hosting platform. See the ultra cool text editor Sublime Text 2, the fortrabbit web GUI, gorgeous Git version and PHP Composer in action.

The tutorial also shows you how to integrate it with the fortrabbit hosting service platform and deploy the Symfony2 site, complete with the correct .htaccess file to handle the rewrites. The commands you'll need for the terminal are all included in the post. You can find out more about the "create-project" feature in Composer in its manual.