Retrieve WordPress root directory path?

Note: This answer is really old and things may have changed in WordPress land since.

I am guessing that you need to detect the WordPress root from your plugin or theme. I use the following code in FireStats to detect the root WordPress directory where FireStats is installed a a WordPress plugin.

function fs_get_wp_config_path()
{
    $base = dirname(__FILE__);
    $path = false;

    if (@file_exists(dirname(dirname($base))."/wp-config.php"))
    {
        $path = dirname(dirname($base))."/wp-config.php";
    }
    else
    if (@file_exists(dirname(dirname(dirname($base)))."/wp-config.php"))
    {
        $path = dirname(dirname(dirname($base)))."/wp-config.php";
    }
    else
        $path = false;

    if ($path != false)
    {
        $path = str_replace("\\", "/", $path);
    }
    return $path;
}

Looking at the bottom of your wp-config.php file in the wordpress root directory will let you find something like this:

if ( !defined('ABSPATH') )
    define('ABSPATH', dirname(__FILE__) . '/');

For an example file have a look here:
http://core.trac.wordpress.org/browser/trunk/wp-config-sample.php

You can make use of this constant called ABSPATH in other places of your wordpress scripts and in most cases it should point to your wordpress root directory.

echo ABSPATH; // This shows the absolute path of WordPress

ABSPATH is a constant defined in the wp-config.php file.