BigCommerce Single Sign On

I have a bigcommerce store as well as a customer portal. There is a sync mechanism already in place for the customers between the two. I want single signin on in my big commerce store, if customer is logged in customer portal. You have to use customer login api provided by BC. Project has to be done in PHP. This is a very simple project as most of the code samples are already provided by BC. I dont want the ingegration in my portal, but a plugin function which I can use.