Symfony 3.3 has redesigned the exception pages to modernize them and match their design with the toolbar and the profiler.
In Symfony 3.3 we optimized the performance of the routing component to make your applications faster, specially when they define a lot of routes.
Symfony 3.3 defines a new Kernel::getProjectDir() to replace Kernel::getRootDir() as the way to find the root project directory.
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.