include_once"/include/a.php" 和include_once" . /include/a.php"有什么区别？解决办法

include_once"./include/a.php" 和include_once". . /include/a.php"有什么区别？？？
include_once"./include/a.php" 和include_once". . /include/a.php"有什么区别？？？

------解决方案--------------------
着是绕口令嘛？
根据引用文件的位置不同，引入文件路径不同，路径不一样自然功能就不一样 
一个是当前目录下的/include/a.php
一个是根目录下的/include/a.php

/include/include/include.php
/include/a.php
/include/include/a.php

如果当前引用的文件在include/include 下的include.php
include_once"./include/a.php" 
 那么它将引用include/include/a.php

include_once". . /include/a.php"
它将引用include/a.php

/include.php
/include/a.php
/include/include/a.php
如果引用是在根目录下 include.php
那么上面两个引用将指向同一个文件. /include/a.php