Laravel\Lumen\Application::storagePath PHP Method

    public function storagePath($path = null)
    {
        return $this->basePath() . '/storage' . ($path ? '/' . $path : $path);
    }

 /**
  * Maintenance file path.
  *
  * @return string
  */
 public function maintenanceFilePath()
 {
     return $this->app->storagePath($this->maintenanceFile);
 }