https://xenby.com/b/305-%e6%95%99%e5%ad%b8-%e5%a6%82%e4%bd%95%e4%bd%bf%e7%94%a8-phpunit-%e9%80%b2%e8%a1%8c%e6%b8%ac%e8%a9%a6%e8%88%87-mock-function