The scripting language that performs the redirect isn't actually PHP. It's Javascript. Usually, when a form field change results in an immediate action without a submit button being pressed, it's Javascript.
In this case, this is the HTML/Javascript
:
Code:
<SELECT NAME="part_id" ONCHANGE="submit()">
The onchange Javascript handler submits the form, which in turn retrieves the corresponding info from the form post and generates the page.
The back-end admin would really depend on what type of product it was, what information you wanted to show, the structure of the database, etc. There's about a million ways to handle this sort of thing, and I don't know of a generic script that will perform the task.