Integrating an internet and intranet php application

Hi all,

I have been working with php/mysql for about a year now. I developed a php inventory system on the public Internet. I have now been asked to create a customer information system that will run in an INTRAnet environment. What php and/or perl functions can I use to seamlessly integrate the intranet and internet applications.

For example:
We have all the product information online, but we want all the customer information offline(especially credit card stuff). When we make a sale, we would like to have the customer information displayed along with the product information. This will be used for invoicing and then for accounting purposes.

By the way, this will be a rather large project. Would anybody be interested in open sourcing it. We could join forces and create a huge system.