首先我们来看php官方手册中对include的文件搜索原则的描述：
Files for including are first looked for in each include_path entry relative to the current working directory, and then in the directory of current script. E.g. if your include_path is libraries , current working directory is /www/ , you includedinclude/a.php and there is include "b.php" in that file, b.php is first looked in /www/libraries/ and then in /www/include/ . If filename begins with ./ or ../ , it is looked only in the current working directory.
寻找包含文件的顺序先是在当前工作目录的相对的 include_path 下寻找，然后是当前运行脚本所在目录相对的 include_path 下寻找。例如 include_path 是 . ，当前工作目录是 /www/ ，脚本中要 include 一个 include/a.php 并且在该文件中有一句 include "b.php" ，则寻找 b.php 的顺序先是 /www/ ，然后是 /www/include/ 。如果文件名以 ./ 或者 ../ 开始，则只在当前工作目录相对的 include_path 下寻找。
所以如下所示的文件结构
----a.php
----include/b.php
----include/c.php
其中a.php
<?php
include 'include/b.php';
?>
-----------------------
b.php
<?php
include 'c.php';
include 'include/c.php';
?>
--------------------------
c.php
<?php
echo 'c.php';
?>
--------------------------
都能正确运行，说明b.php中两种不同包含路径都是可行的，根据include寻找包含文件的方式都能找到c.php。
但是最好的方式还是使用绝对路径，如果使用了绝对路径，php内核就直接通过路径去载入文件而不用去include path逐个搜索文件，增加了代码执行效率
<?php
define('ROOT_PATH',dirname(__FILE__));
include ROOT_PATH.'/c.php';
?>
不同的文件包含方式，程序的执行性能比较具体可以参照此文