[calorie]

You can do PHP includes in products/plugins. Example:

Hook location:
global_start

PHP code:
require_once('/path/to/file.php');

Here is an example if your PHP code prints output:

Hook location:
global_start

PHP code:
ob_start();
require_once('/path/to/file.php');
$some_output = ob_get_contents();
ob_end_clean();

Use global_start only if you want the code to run on most pages.
Otherwise, choose a more appropriate hook location.

More information can be found at the following links:

• http://www.vbulletin.com/docs/search?q=plugin
• http://www.vbulletin.com/docs/search?q=product