So you have an application in your web space, with a URL such as this:
http://<host>/<installation_path>/
and pages such as
http://<host>/<installation_path>/subfolder1/subfolder2/page.php
You have a file called config.php in <installation_path> which is include()d by all pages (in subfolders or not).
How to work out <installation_path> without hard-coding it into a config file?
<?php
// this is config.php, and it is in <installation_path>
// it is included by <installation_path>/page.php
// it is included by <installation_path>/subfolder/page2.php
// etc
$_REAL_SCRIPT_DIR = realpath(dirname($_SERVER['SCRIPT_FILENAME'])); // filesystem path of this page's directory (page.php)
$_REAL_BASE_DIR = realpath(dirname(__FILE__)); // filesystem path of this file's directory (config.php)
$_MY_PATH_PART = substr( $_REAL_SCRIPT_DIR, strlen($_REAL_BASE_DIR)); // just the subfolder part between <installation_path> and the page
$INSTALLATION_PATH = $_MY_PATH_PART
? substr( dirname($_SERVER['SCRIPT_NAME']), 0, -strlen($_MY_PATH_PART) )
: dirname($_SERVER['SCRIPT_NAME'])
; // we subtract the subfolder part from the end of <installation_path>, leaving us with just <installation_path> :)
?>