Drupal - Add a Directory to Your Site's Include Path

Say for example, one of your Drupal modules needed to access some PHP function located in a PHP file in a directory somewhere on your server that wasn't in your Drupal directory.

/my_custom_directory/my_custom_php_library/my_custom_php_library.php

You can make Drupal aware of that directory by adding it to your site's include path. In your settings.php file for your Drupal site, add something like this:

ini_set('include_path', ini_get('include_path') . ':/my_custom_directory/my_custom_php_library');

Now, just add a call to require_once in your module to load up your custom php file:

require_once("my_custom_php_library.php");

Now any functions located in your custom php file will be available to your Drupal module.