Laravel Form Validation combination of first and last name must be Unique

UserController.php

public function store(Request $request)
{
    $validator = Validator::make($request->all(), [
        'first_name' => 'required|max:100',
        'last_name'  => 'required|unique:user,last_name,NULL,id,first_name,'.$request->first_name.'|max:100',
    ]);

    ....
}