Do you know all those technologies? If not then start by forgetting about AJAX and XML (for now), you only need those if you want to have a dynamic page that updates without reloading.
I'd start with a single page with a form on it to allow the user to enter the ID; POST the data back to the same form and include a PHP script that, if there is POST data available uses it in a query to the database and displays the result below the form.
Remember to use bast practices in preventing SQL injection attacks, start as you mean to go on with a secure solution.