1、__FILE__
__FILE__ always equals to the real path of a php script regardless whether it's included.
__FILE__ helps you specify the file to include using relative path to the including file.
这种方法首选推荐。虽然你的include语句会因此要写得长一些，但是一个字，值！

<?php
include dirname(__FILE__).'/subdir'; 
//dirname return value does not contain the trailing slash 
?>
 

2、$_SERVER['DOCUMENT_ROOT']
This method allows you to specify a path relative to the web server doc_root for file inclusion.
这也是许多项目在采用的一种不错的方式，就我看来，缺点是，整个项目不方便移动。

例如你一开始放置在xxx.com/，后来需要放到xxx.com/abc/下的话，你要改文件（在一个公有文件中计算ROOT的位置，其他文件包含这个共有文件）。
特别是当你同一份代码放多处时（例如一个测试环境和一个正式环境），你改文件也不好改。
<?php
if (!defined("WETSITE_BASE_DIR"))
define("WETSITE_BASE_DIR", $_SERVER['DOCUMENT_ROOT'].'/Clare/');
require_once(WETSITE_BASE_DIR.'includes/global.inc.php');
?>

3、chdir()
The include looks for file relative to current working directory. We can use this feature. It's really a "fancy" way, but I'm not sure whether it's safe all the time. Who knows?
这种方式感觉稍嫌麻烦了点，随时要记得恢复工作目录也不是容易的事。写完这句话后，我随后写了几个测试文件，发现这种方式的最重要缺点不在麻烦，而在它的副作用：改变了工作目录，这会导致程序逻辑出错。

rainfalling at yahoo dot com (21-Sep-2005 01:06)
This is yet another way to include files relative to the current file. I find it easier if you have a lot of includes.

<?php 
$prewd = getcwd(); // get the current working directory
chdir(realpath(dirname(__FILE__))); // change working directory to the location of this file 
include('includedfile.php'); // include relative to this file 
chdir($prewd); // change back to previous working dir
?>

4、set_include_path()

This way is the most convenient way but it's not without flaws. First, not in all cases you have permission to change server configuration. Second, if there are many path specified in include_path, the actually included file may not be the one you expected because there may be files of the same name under different directories.
这是最方便的方式，但不是没有缺点。首先，有时候你不见得有权限修改配置。其次，当不同路径下的文件名有重复的时候，你会被搞糊涂的（就算你不会，你的维护者呢）。

5、auto_prepend_file and auto_append_file in php.ini

This almost the best way if your scripts commonly need a startup script. We can do a lot of useful things in the startup script, for examples, define constants, load configurations. But it's not always OK to change the php.ini settings. Remember the most adaptive application should be as independent from configs as possible. 
如果你每个脚本都需要包含一个通用脚本的话，这几乎是最好的方式，但是，缺点还是，与配置相关，不够独立。