Please use [php], [html] or [code] tags (as appropriate) to separate and format code.
PHP checks every directory listed in the include_path directive when searching for include files, unless an absolute (beginning with a "/") or relative (beginning with "./" or "../") path is used. If the file can't be found, PHP will then check the current working directory. The current working directory starts out as the directory of the top-level script (the one the URL maps to, $_SERVER['SCRIPT_FILENAME']).
Use get_include_path and set_include_path to manage the include path. You can do this in a site-wide initialization file (named e.g. "init.php"), then create a per-directory file with the same name in subfolders to include the site-wide file. Start each script with a require_once('./init.php');.
Per-directory init.php:
PHP Code:
<?php
require_once($_SERVER['DOCUMENT_ROOT'] . '/init.php')
With this approach, libraries can be placed outside the web hierarchy, as is proper (library scripts shouldn't be publicly addressable/accessible).
Personally, I prefer to reserve scripts with "config" in their name for admin-editable settings. That is, "config.php" and the like contain only variable assignment statements, whereas "init.php" contains more complex code that only developers should edit.
One way to be certain that the files are being included is to use require_once rather than include_once, which will result in a fatal error rather than a warning if the file isn't found. Note that you should only do this if a missing included script will prevent the including script from doing anything useful.
As for sending e-mails, make sure your code performs proper error checking at all potential failure points. The built-in mail function offers very little when it comes to reporting e-mail problems; use a more full-featured PHP mailer, such as PEAR's Mail or PHPMailer to get better feedback.
<br/> and <font> arn't semantic. Similarly, don't use tables for layout, as it's not semantic. Instead of <br>, use a more appropriate element, such as <p>. Instead of <font> and table-based layouts, use CSS. Make sure any element class names you use are also semantic.
Don't use exit or die when outputting HTML. You'll get invalid HTML.