What is the difference between Symfony 5 and 6?

Symfony 6 is based on Symfony 5 and brings more performance improvements, development simplifications, and modernization of code practices. Symfony 6 necessitates the use of PHP 8 and, correspondingly, supports new language features such as attributes. Symfony 6 does not support the deprecated features in Symfony 5, which means tidier and more productive code. It promotes new tools, such as the Notifier component, and maintains a core presence in performance optimization and PHP compatibility. As of now, Symfony 6 is future-proof for modern applications, while Symfony 5 is good enough to deal with supporting older PHP versions.