Hello everyone.

Recently I acquired a new client that has me doing web development for them -- pretty basic stuff, integrating an existing web design with WordPress. However, my client recently approached me about setting up a system whereby they would have the ability to allow customers to securely create an username and password with an existing account number, log-in and view/pay their bills.

Now, I am very experienced with most aspects of web development (php, sql, JavaScript, etc.) and have A LOT of experience with CMSs like WordPress, Joomla and their ilk, but I do not have much experience with systems like this. Moreover, I know enough about security and encryption to know that trying to implement my own system in something this complex and important has the potential for disaster.

Therefore, I am wondering if anyone knows of any good CMS that could be employed to meet the above requirements. Also, keep in mind that my client is a small-town, medium-sized energy provider and needs a scalable solution (currently they have about 20,000 customers) They also have a relatively complex billing situation and would be willing to pay for another company to maintain a platform making the above possible.

Any suggestions are welcome!