I would like to point out the difference in behavior in IIS/Windows and Apache/Unix (not sure about any others, but I would think that any server under Windows will be have the same as IIS/Windows and any server under Unix will behave the same as Apache/Unix) when it comes to path specified for included files.

Consider the following:
<?php
include '/Path/To/File.php';
?>

In IIS/Windows, the file is looked for at the root of the virtual host (we'll say C:\Server\Sites\MySite) since the path began with a forward slash.  This behavior works in HTML under all platforms because browsers interpret the / as the root of the server.

However, Unix file/folder structuring is a little different.  The / represents the root of the hard drive or current hard drive partition.  In other words, it would basically be looking for root:/Path/To/File.php instead of serverRoot:/Path/To/File.php (which we'll say is /usr/var/www/htdocs).  Thusly, an error/warning would be thrown because the path doesn't exist in the root path.

I just thought I'd mention that.  It will definitely save some trouble for those users who work under Windows and transport their applications to an Unix-based server.

A work around would be something like:
<?php
$documentRoot = null;

if (isset($_SERVER['DOCUMENT_ROOT'])) {
    $documentRoot = $_SERVER['DOCUMENT_ROOT'];
    
    if (strstr($documentRoot, '/') || strstr($documentRoot, '\\')) {
        if (strstr($documentRoot, '/')) {
            $documentRoot = str_replace('/', DIRECTORY_SEPARATOR, $documentRoot);
        }
        elseif (strstr($documentRoot, '\\')) {
            $documentRoot = str_replace('\\', DIRECTORY_SEPARATOR, $documentRoot);
        }
    }
    
    if (preg_match('/[^\\/]{1}\\[^\\/]{1}/', $documentRoot)) {
        $documentRoot = preg_replace('/([^\\/]{1})\\([^\\/]{1})/', '\\1DIR_SEP\\2', $documentRoot);
        $documentRoot = str_replace('DIR_SEP', '\\\\', $documentRoot);
    }
}
else {
    /**
     * I usually store this file in the Includes folder at the root of my
     * virtual host. This can be changed to wherever you store this file.
     * 
     * Example:
     * If you store this file in the Application/Settings/DocRoot folder at the
     * base of your site, you would change this array to include each of those
     * folders.
     * 
     * <code>
     * $directories = array(
     *     'Application',
     *     'Settings',
     *     'DocRoot'
     * );
     * </code>
     */
    $directories = array(
        'Includes'
    );
    
    if (defined('__DIR__')) {
        $currentDirectory = __DIR__;
    }
    else {
        $currentDirectory = dirname(__FILE__);
    }
    
    $currentDirectory = rtrim($currentDirectory, DIRECTORY_SEPARATOR);
    $currentDirectory = $currentDirectory . DIRECTORY_SEPARATOR;
    
    foreach ($directories as $directory) {
        $currentDirectory = str_replace(
            DIRECTORY_SEPARATOR . $directory . DIRECTORY_SEPARATOR,
            DIRECTORY_SEPARATOR,
            $currentDirectory
        );
    }
    
    $currentDirectory = rtrim($currentDirectory, DIRECTORY_SEPARATOR);
}

define('SERVER_DOC_ROOT', $documentRoot);
?>

Using this file, you can include files using the defined SERVER_DOC_ROOT constant and each file included that way will be included from the correct location and no errors/warnings will be thrown.

Example:
<?php
include SERVER_DOC_ROOT . '/Path/To/File.php';
?>