Facebook SKD 4.0 example on Symfony 2.5

how_to_use_facebook_symfony2

###PHP requirements to be able to use php SDK
- php-mbstring

In Centos you can install this php-module like this:
```bash
$ yum install php-mbstring
```

###Symfony changes required:
In your config.yml need to change the session handler

```yaml
framework:
    session:
        # handler_id set to null will use default session handler from php.ini
        handler_id: session.handler.native_file
        save_path: "%kernel.root_dir%/sessions"
```

Don't forget that to use facebook sdk you need to set: `session_start();` before all your scripts.