Cloud based software

We're currently working on a SaaS. We're starting from scratch and with not a lot of knowledge and experience. However, we have a very clear idea of how our software looks like and we're doing tons of research on this topic. We've decided to focus on the user experience design, and the front end in general. We're going to use jQuery, PHP, Ajax, HTML5, CSS3 for the front end, and a MySQL database. Now, it's the back end what we want you to develop. This is a cloud-based service, so we need a very efficient application when it comes to bandwith usage. It needs to be responsive and accurate, as it will make predictions, suggestions, etc. This is an application for the business environment... with a twist. We want a relatively simple back end that can power some of the basic features of the application.