How to pass variables to the routing file using laravel / lumen?

At the bottom of bootstrap/app.php there's the following code to include the routes file...

$app->router->group([
  'namespace' => 'App\Http\Controllers',
], function ($router) {
    require __DIR__.'/../routes/web.php';
});

$router is now available within the required web.php file, which makes sense, however I don't see how or where $router was created to be passed in to the anonymous function... hope that makes sense... thanks to any responders.

Solution

In the source code for the Router class, the Router invokes the anonymous function with an instance of itself as a parameter hence the $router in the anonymous function.