Hi,
the "user picks [...]" is a client side event.
So you can't do this just with php, that is server side and
don't know what happens on the client machine before it post the page.
So you need something like javascript.
You link any function to the ONCHANGE event of <select> and when the user picks a value this will reload the page passing the value. Now you can fill all field of page using the selected value.
This is the more PHP and less JAVASCRIPT way, I think.
(It's enough to do
<select ONCHANGE="document.location='<?=$PHP_SELF?>&sel='+this.value">
and when the page reload you can get the selected value in $sel or in $_GET['sel']...
)
I hope this helps you...