include and require will look for a file relative to the setting given to it in php.ini first and foremost.
Say your ini file's include path entry is:
include_path = "var/www/includes;/var/www/PEAR"
Then in your scripts, no matter where in your document tree they are, eg
/var/www/html/website1/miles/down/in/folders/index.php
you just do this to include a file:
include 'settings.php' ;
As long as settings.php is one of the include_path folders, it will be included, then you can stop worrying about relative/absolute path relationships.
This setting can be altered in .htaccess files and per-file using ini_set() if you want too.
More on this:
http://php.net/manual/en/function.set-include-path.php
http://www.modwest.com/help/kb.phtml?cat=5&qid=98
or google for "include_path php"