$app->get('/books/{id}', function ($id) use ($app) { $book = $app['db']->fetchAssoc('SELECT * FROM books WHERE id = ?', array((int) $id)); return json_encode($book); });In this example, we create a route that corresponds to the `/{id}` URL. The `$id` parameter can be any integer value, and it's passed to the controller function through Silex's dependency injection system. Inside the function, we use the `$app` object to access the database service (`$app['db']`) and retrieve the book with the corresponding `$id`. Finally, we encode the book as JSON and return it as the response. The `fetchAssoc` method is not part of Silex, but rather a part of Doctrine DBAL library, which can be integrated with Silex using the `DoctrineServiceProvider`. In summary, Silex is a PHP micro-framework that allows you to handle various HTTP requests, and get data using the `get` method. Furthermore, it has integration with several packages, such as the `DoctrineServiceProvider`.