Plain session docs – This is the symfony documentation page about Sessions alone. This link shows the basic configuration and use of Sessions in Symfony. This also mentions not starting a session for Anonymous users and has links to other info about sessions.
Configuring Sessions and Save handlers – Symfony documentation link. This covers more about how to configure sessions and their Save Handlers. This is some of the better information about Sessions and how they work in Symfony. It covers the save handlers and more of the configuration information.
Session proxy examples – Symfony Documentation link. This covers how to create your own session handler. It also discusses how to encrypt session data with an example.
Framework configuration – Symfony documentation link. This covers many of the options for the security component of Symfony.
Store sessions in a database – Symfony documentation link that describes how to store session data in a database or Redis.
Session Management – Symfony documentation link. Explains how sessions are managed in symfony. Gives a good overview and important information about how symfony functions. It covers the functions symfony uses to replace PHP session functions and how to use them. This also covers the ways to work with sessions in Symfony. Oddly this covers Flash messages too.