public static function getPDO($dsn, $username, $password, $settings = ['attributes' => []])
    {
        // if not set self pdo object property or pdo set as null
        if (!isset(self::$PDO) || (self::$PDO !== null)) {
            self::$PDO = new self($dsn, $username, $password, $settings); // set class pdo property with new connection
        }

        // return class property object
        return self::$PDO;
    }