I try to include my files, but for some reason I can’t…
Here is my structure:

On index.php, there is a include_once('includes/php/inc.php');. The inc.php contains these lines:
// Required classes
require_once '../classes/cl_calendar.php';
Perfect legitimate I think, but for some reason all I get is this error:
Warning: require_once(../classes/cl_calendar.php): failed to open stream: No such file or directory in /customers/xxx/xxx/httpd.www/new/extra/php/inc.php on line 14 Fatal error: require_once(): Failed opening required '../classes/cl_calendar.php' (include_path='.:/usr/share/php') in /customers/xxx/xxx/httpd.www/new/extra/php/inc.php on line 14
What makes this error to be displayed and how do I get rid of it?
The correct path is includes/classes/file.php. Why the ..? This would lead you one level over the root.
The reason for that is that even though the relative relation of the two included files may be a different one, you’re including it from the index file and therefore the path has to be relative to the index file.