https://blog.wongcw.com/2018/11/16/php-%e4%bd%bf%e7%94%a8-%e6%93%8d%e4%bd%9c%e7%ac%a6%ef%bc%8carray_merge-%e5%87%bd%e6%95%b8%ef%bc%8c%e4%bb%a5%e5%8f%8aarray_merge_recursive-%e5%87%bd%e6%95%b8%e9%80%b2%e8%a1%8c%e5%90%88%e4%bd%b5/