In Symfony 3.3, it's easier to work with flash messages in templates. In addition, the session no longer starts automatically when checking if there are flash messages.
In Symfony 3.3, the Workflow component will include lots of useful new features.
In Symfony 3.3, the special SYMFONY__ environment variables have been deprecated and they will no longer work on Symfony 4.0. Use regular environment variables instead.
Symfony 3.3 adds a new command called "about" to display useful information about your Symfony application and your PHP environment.
Symfony 3.3 adds a new versioning strategy based on JSON file manifests, which is used by popular tools such as Webpack.
Symfony 3.3 deprecates the ConsoleEvents::EXCEPTION event in favor of a new ConsoleEvents::ERROR event that is able to handle also the exceptions thrown from listeners.
Symfony 3.3 adds a new Lock component that supports different stores (flock, semaphore, Redis, Memcache), auto-expiring locks and combining multiple lock stores.
Symfony 3.3 adds full support for loading config files using glob patterns, both for the container and the routing file loaders.
Symfony 3.3 deprecates the warmup part of the cache:clear because it does not work well and it doesn't guarantee that the generated cache is exactly the same as the one that would have been generated via cache:warmup.
Symfony 3.3 adds new features to simplify services configuration thanks to the default config options and the interface-based config options.