https://blog.wongcw.com/2018/11/16/php-%e4%bd%bf%e7%94%a8%e9%9d%9c%e6%85%8b%e8%ae%8a%e9%87%8f%ef%bc%88static-var%ef%bc%89%e9%80%b2%e8%a1%8c%e6%95%88%e7%8e%87%e5%84%aa%e5%8c%96/