What is Symfony
Symfony is a set of PHP Components, a Web Application framework, a Philosophy, and a Community — all working together in harmony.
Symfony Framework:

The leading PHP framework to create websites and web applications. Built on top of the Symfony Components.
Symfony Components: A set of decoupled and reusable components on which the best PHP applications are built, such as Drupal, phpBB, and eZ Publish.
Symfony Community: A passionate group of over 600,000 developers from more than 120 countries, all committed to helping PHP surpass the impossible.
Symfony Philosophy: Embracing and promoting professionalism, best practices, standardization and interoperability of applications.
Official site is https://symfony.com/