$_SERVER的有关问题，小弟我说为什么一直url重写失败，去不掉index.php

$_SERVER的问题，我说为什么一直url重写失败，去不掉index.php
有的主机url重写后
$_SERVER['PATH_INFO']会消失，
取而代之的是
$_SERVER["ORIG_PATH_INFO"]
从而导致，根据$_SERVER['PATH_INFO']判断来去掉index.php会失效
我只是发现了这个现象，却不明白原理是为什么？
------解决方案--------------------
PATH_INFO is missing but ORIG_PATH_INFO is there with the information PATH_INFO was supposed to have.

So the php code had to be changed to check for $_SERVER['ORIG_PATH_INFO'] when $_SERVER['PATH_INFO'] is absent

见 http://www.binarytides.com/path_info-orig_path_info-apache-and-php/ 最后一段