While their sample code is not, the development documentation is publicly available on their website nowadays: http://docs.whmcs.com/Provisioning_Module_Developer_Docs So I do not see the issue about it being confidential. The main problem with closed source modules is that they tend to come with bugs, which can only be found and properly solved by the original developer (you) and not by others. In fact your module currently has an issue, which causes other WHMCS modules stop working if the Plesk module is installed. Your module includes a hooks.php that is always loaded by WHMCS, even when the product the user is viewing is not Plesk related. I suspect you do something that loads the Zend framework there -that you bundled with your module- and this causes problems for other modules that bundle their own version of the Zend framework as well. They can no longer load their version, because yours is already in loaded, and you cannot have a class with the same name multiple times. But that is just an educated guess, given I cannot view your source.