How to load an include file from Drupal's include folder?
require_once DRUPAL_ROOT . '/includes/actions.inc';


is the correct way.

Using 'content' just worked, because there is no module named content and just did work due to a side-effect. It also caused silent performance problems on Drupal versions &lt; 7.50.

There is no Drupal specific function to load a core include file, and this is how it is done in includes/common.inc line 5242.


Fabianx, Drupal 7 core branch maintainer and core committer