PHP Javascript data passing

I'd like code to take data from a mysql database and then pass that to a Javascript array and then from the array back to the database. Ideally I'd like to pass the data using AJAX and have the server side written in PHP. I've thought of somthing like. PHP retreives data from MYSQL translates it to JSON or XML, file retrieved by Javascript function which puts data into an array which will be an object. I'm open to suggestions about the best method and architecture. I want something quick I don't want to use libs that are on the net, I want it from scratch. I think if you know what you're doing this isn't such a big job. I'm looking for a demo system that works passing data backwards and forwards. Doens't have to look fancey just enough of a hanress to demo that the system works. You need to document the scripts fully as well.