Get the referral source url in Laravel

I am a Laravel-newbie and have got the following piece of code in my landing page:

Route::get('/prelaunch', [ 'uses' => 'SubscriptionController@getReferrer', 'as' => 'subscriber.referral'], function () {
    return view('prelaunch');
});

What I would like to do, is to read a field such as "utm_source" from the url and store take some actions against it, such as counting the users that land into the page from twitter, fb etc.

Any suggestions on this one?

via Chebli Mohamed