you need to include the page name in the value of the select list.
change:
<option value="<?php echo $row_rsTest['clientID']?>"><?php echo $row_rsTest['client']?></option>
to:
<option value="pagename.php?clientID=<?php echo $row_rsTest['clientID']?>"><?php echo $row_rsTest['client']?></option>
where pagename.php is the name of the page that should be loaded.