Laravel: Get variable name from route

How can I get the name of the get variable from a route url?

http://localhost:8000/api/system/123

Basically I want to know what the GET variable is called that holds the value of 123. For example here it is called id

http://localhost:8000/api/system?id=123

How can this be done?

Route::get('api/system/{id}', 'ApiController@User');

via Chebli Mohamed