PHP novice having problems with a super simple image swap thingie

Hi, and sorry to trouble you with something this simple. ;(

I’m doing a design portfolio with lots of images.

The basic layout is: large image on the left side, thumbnails for other views on the right side.

How can I make a dynamic page that updates the large image when you click on thumbnails?

What I’ve done so far is:

switch ($id) {
case "client1_01":
$client_img = "img_client1_01.jpg";
case "client1_02":
$client_img = "img_client1_02.jpg";
$client_img = "img_client1_01.jpg";

HTML code that shows the large image:

<img src="images/<?php echo $client_img ?>" alt=""/>

HTML link syntax to update the large image:

<a href="?id=client1_01"><img src="images/img_marker_90x70.gif"/></a>

These are all in one file, index.php.

It appears that when I click on a link and the url refreshes with the new var content (eg index.php?id=client1_01) it always runs the default case. Why is that?

Thank you so much,