This document discusses Symfony 3 migration and changes from previous versions. Key points include:
- Symfony 3 will focus on fixing architectural mistakes and removing deprecated code rather than major changes.
- The release process follows semantic versioning and backward compatibility breaks are allowed for major releases.
- To migrate, applications using versions before 2.3 should upgrade to 2.3 first, and those between 2.3-2.6 should upgrade to 2.7/2.8.
- Third party bundles may also need to be upgraded to support Symfony 3.
36. Make your code deprecation free
- fix your code
- update vendors if needed
http://symfony.com/doc/current/cookbook/upgrade/index.html
https://knpuniversity.com/blog/upgrading-symfony-2.7