pack 文档：https://www.php.net/manual/en/function.pack.php
unpack 文档：https://www.php.net/manual/en/function.unpack.php
PHP中pack、unpack的详细用法：https://segmentfault.com/a/1190000008305573

在线示例：https://sandbox.onlinephpfunctions.com/c/56feb