Some excerpts from the Official Symfony Best Practices with example code.

Always use Composer to install Symfony.

composer create-project symfony/framework-standard-edition blog/

Create only one bundle called AppBundle for your application logic

Define the infrastructure-related configuration options in the app/config/parameters.yml file.

Define all your application's parameters in the app/config/parameters.yml.dist file.

Define the application behavior related configuration options in the app/config/config.yml file.

Use constants to define configuration options that rarely change.

The name of your application’s services should be as short as possible, but unique enough that you can search your project for the service if you ever need to.

Use the YAML format to define your own services.

Use annotations to define the mapping information of the Doctrine entities.

Symfony follows PSR-1 and PSR-2!

Make your controller extend the FrameworkBundle base Controller and use annotations to configure routing, caching and security whenever possible.

Don't use the @Template() annotation to configure the template used by the controller.

Use the ParamConverter trick to automatically query for Doctrine entities when it's simple and convenient.

Use Twig templating format for your templates.

Store all your application's templates in app/Resources/views/ directory.

Define your Twig extensions in the AppBundle/Twig/ directory and configure them using the app/config/services.yml file.

Define your forms as PHP classes.

Add buttons in the templates, not in the form classes or the controllers.

Use the XLIFF format for your translation files.

Store the translation files in the app/Resources/translations/ directory.

• For protecting broad URL patterns, use access_control;
• Whenever possible, use the @Security annotation;
• Check security directly on the security.context service whenever you have a more complex situation.

Store your assets in the web/ directory.

Define a functional test that at least checks if your application pages are successfully loading.

Hardcode the URLs used in the functional tests instead of using the URL generator.