You'll have two links at the top of the column: asc | desc
<a href="sort.php?o=asc">asc</a> | a href="sort.php?o=desc">desc</a>
sort.php simply saves their choice as a cookie and returns back to the list.
Each time the list is displayed, it looks at the sort cookie and depending on
which one they pick, the script will change the ORDER BY in the query.