I need to refer to an external class in a custom process, how do I do that?

Environment:

Process Designer, Custom Process Model (CPM) / Service Process Model (SPM)
November 2012 version and newer

Resolution:

External classes will require special care in custom process. As the custom process has its own namespace, users need to break out of it to reference a class that is defined at the root level.

To explicitly reference the root scope:

$cfg_obj = new \CTK_CONFIG('lithium');

You need to have the correct included path: Answer ID 6707: Include paths for custom process 

...and to have successfully completed a require_once('ctk/classes.php') in this example