By default, Backbone.js and its synchronization method can work with the REST API without any changes. Therefore, if you can create a web service with Symfony2 that can handle GET / POST / PUT / DELETE and return JSON, you should be good to go.
I'm not very familiar with Symfony, but for the basic REST API you need to define URL routes (for example, "www.yourdomain.com/api/items") and then create methods to perform actions based on these routes (for example, entries return in JSON encoded format).
Here's a quick guide to creating a REST API with Symfony2: Symfony2 REST
Also, check out the Wine Cellar tutorial on the Backbone tutorial page. It uses the PHP Slim framework, but the concepts of backbone.js will be exactly the same because, as long as you use the basic REST API, the backbone is agnostic to the back-end.
eterps
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