Symphony CMS is a content management system that is built using PHP language. Despite the similarity in naming it has nothing to do with the Symfony project also written in PHP.
The Symphony CMS is a versatile Open Source tool for managing content. Unlike many familiar LAMP CMS like WordPress, Symfony uses XML for content storage. This content is then transformed to HTML using XSLT for presentation to website visitors. The model allows rich content structures for chunky content and more.
Symphony has been in development for a number of years and it is very stable and mature product. It is used by household brands, but the age is showing in the codebase as it is not very modern PHP. A gradual rewrite process can now be started as the project practises are contemporary with code in GitHub and semantic versioning in place.