Request::header('app-version');

use App\Http\Controllers\Api\UserController as UserCtrl;
class UserController extends UserCtrl
{
  ...
}

function getRouteContoller($controllerName)
{
  $base_path = 'App\Http\Controllers\Api\\';
  //無版本
  $app_version = Request::header('app-version');
  if (!$app_version) return $controllerName;
  //當前版本
  $version_path = 'v' . str_replace('.', 'd', $app_version) . '\\';
  $controllerPath = $base_path . $version_path . $controllerName;
  if (class_exists($controllerPath)) {
    return $version_path . $controllerName;
  }
  //往期版本
  $versionNumbers = intval(str_replace('.', '', $app_version));
  for ($i = $versionNumbers - 1; $i > 0; $i--) {
    $version_path = 'v' . implode('d', str_split($i)) . '\\';
    $controllerPath = $base_path . $version_path . $controllerName;
    if (class_exists($controllerPath)) {
      return $version_path . $controllerName;
    }
  }
  return $controllerName;
}

Route::post('login', getRouteContoller('UserController').'login');