Laravel: Passing custom variables to a controller from a route

I have the following code in my routes file:

$domain1 = 'www.domain1.co.uk';

Route::group(array('domain' => $domain1), function() use ($domain1)
{
    Route::get('/', 'PrimaryController@index');
});

How can I pass the variable $domain1 to the controller method?

To expand to a more general question, how can I simply declare some variable in my routes file and then pass it to a controller? I know I can do this:

Route::get('tours/{id}/{title}', 'PrimaryController@tourHandler');

...and have access to the $id and $title variables in the controller- Which is great if those variables are present in the URL. But what if, for whatever reason, I want access to some specific variables which I want to set myself in the routes file?

Laravel seems too clever sometimes.