Symfony is a set of PHP components and a full stack framework. It's commonly used by many content management systems powering large parts of the web. Here are a few options for developers to work with.
eZ Platform is a CMS built on the Symfony Framework. It is powerful and a total rewrite of the eZ Publish system in development from 1999.
Bolt CMS is a lightweight CMS built with the Silex Microframework. It is handy and a great alternative for developers looking for a modern PHP to replace WordPress, for example.
Drupal 8 is a partial modernisation of the legacy Drupal CMS with some parts of Symfony.