Integrating with cPanel from PHP using their Single Sign On functionality

General information for the website: Web Hosting Company
Kind of development: Customization of existing website
Num. of web pages/modules: 2
Description of every page/module: I currently use WHM and cPanel and offer Reseller Hosting to my clients via the website brixly.uk

I am in the process of building a 'Reseller Area' within WHMCS where I have used the WHM API to pull in a list of sites that belong to a particular reseller account.

What I would like to do is provide a link or button that will use the cPanel / WHM API to sign a user into cPanel to manage their hosting account. The account password isn't always known, so the single sign on functionality must be used to leverage the resellers / owners login details. I have already started the code based on the example provided on the documentation, however it doesn't appear to be working.

I would also like an additional option that will automatically sign the user into WHM.

I would love for someone to guide me through the process, however if this isn't possible I would settle for just getting it completed.

This is also an urgent requirement.
Description of requirements/features: Please see the description above.

cPanel Login using SSO
CMS and Admin requirements: WHMCS Experience / Smarty Templates
cPanel WHM Integration
Extra notes: