This is just a small helper for anybody struggling, with WHMCS development. It contains the definitions for majority of the WHMCS core classes, therefore making a WHMCS development a bit less painful experience :)
Just copy the whmcs_ide.php file somewhere into your project, should work fine with most of them like PHPStorm, VSCode, Atom, NetBeans etc.
Alternatively you can use composer:
composer require indicio-software/whmcs-ide-helper
As long as your vendor directory is not excluded from indexing, you should see all the definitions in place :)