PHP and Server-Side XML reasonable solution for simple number-tracking app?

[starpuff]

I need to write an app that allows any number of web clients to increment and decrement a number. This, along with identifying info, gets sent to and stored on a server. Concurrently, another app watches all updates from the clients and visualizes that data on another webpage.This kind of problem is new to me, so I'm trying to figure out the simplest way to prototype it. It looks like using client-side javascript to write to server files via ajax might be a bit tricky, so I thought of PHP since it can write to server-side files. Clients would write to an XML file and another app would read it and visualize the results. I also thought about node.js, but it looks like a steeper learning curve.I'm probably looking at about 100 max concurrent clients. I want the data to persist so that I could revisualize a previous moment in time (or restart a crashed service w/o loosing data).Is this a crazy way to do this? I'm guessing the use of a database would be the most common solution but I've never worked with DBs myself. If there's another solution that seems more appropriate, I'm open to that too.