Loading included files the Drupal way

The usual PHP functions include, require, include_once, and require_once should not be used in Drupal to load files from modules that aren't your own. Instead, use module_load_include():

http://api.drupal.org/api/function/module_load_include/6

See also http://api.drupal.org/api/function/drupal_load/6

Even in modules that are your own, while it should be safe to load a file right next to yours, if you are splitting a module into separate files you should use the inclusion

Note that because of this enhanced ability to conditionally load additional files, when re-using functions from other modules in your code, you may well have to load the file the function is in first.

Example:

<?php
   module_load_include('inc', 'biblio', 'biblio.import.export');
?>

Resolution