Description:
------------
from http://cn.php.net/manual/en/function.md5.php
raw_output:
If the optional raw_output is set to TRUE, then the md5 digest is instead returned 
in raw binary format with a length of 16.
It is confusing.
The raw binary format should be writen 16 bytes.
For the Chinese translation, "a length of 16" has been translated into 16 bits. it 
should be bytes.

The original sentence(Chinese translation):
那么 MD5 报文摘要将以原始的 16 位二进制格式返回。