private function loadEnvironment()
    {
        /**
         * Application Environment
         */
        $required_env = [
            # application configuration
            'APP_NAME', 'APP_ENV', 'APP_KEY',

            # database configuration
            'DB_HOSTNAME', 'DB_PORT', 'DB_USERNAME', 'DB_PASSWORD', 'DB_NAME',

            # application mode
            'APP_MODE'
        ];

        $root = $_SERVER['PWD'];

        $dotenv = new Dotenv($root);
        $dotenv->overload();
        $dotenv->required($required_env);
    }