私は誰かのために在庫システムをやっています。テーブル ヘッダーのリンクをクリックして、PN と ASC で並べ替えられるようにしたいと考えています。次に、PN をもう一度クリックすると、DESC で表示されます。しかし、説明で注文して同じこともしたいです。これまでの私のコードは次のとおりです。方向(ASC、DESC)を簡単に入れ替える方法がわかりません。
            if (!isset($cd))
        {
            $cd = 0;
        }
        if (isset($_SESSION['direction']) && $cd == 1)
        {
            if ($_SESSION['direction'] == 'DESC')
            {
                $_SESSION['direction'] = 'ASC';
                $cd = 0;
            } elseif ($_SESSION['direction'] == 'ASC') 
            {
                $_SESSION['direction'] = 'DESC';
                $cd = 0;
            }
        } else
        {
            $_SESSION['direction'] = 'ASC';
        }
        if (isset($_REQUEST['sort']))
        {
            if ($_REQUEST['sort'] == 'pn')
            {
                $sql=mysql_query("select * from inventory ORDER BY pn {$_SESSION['direction']}");
            } elseif ($_REQUEST['sort'] == 'description') {
                $sql=mysql_query("select * from inventory ORDER BY description {$_SESSION['direction']}");
            } elseif ($_REQUEST['sort'] == 'wholesale') {
                $sql=mysql_query("select * from inventory ORDER BY wholesale {$_SESSION['direction']}");
            } elseif ($_REQUEST['sort'] == 'list') {
                $sql=mysql_query("select * from inventory ORDER BY list {$_SESSION['direction']}");
            } elseif ($_REQUEST['sort'] == 'stock') {
                $sql=mysql_query("select * from inventory ORDER BY stock {$_SESSION['direction']}");
            } elseif ($_REQUEST['sort'] == 'location') {
                $sql=mysql_query("select * from inventory ORDER BY location {$_SESSION['direction']}");
            }
        } else {
            $sql=mysql_query("select * from inventory ORDER BY pn {$_SESSION['direction']}");
        }      
        echo "<center><table class=\"myTable\">
        <th><a href=\"inventory.php?mode=list&sort=pn\">PN</a></th><th><a href=\"inventory.php?mode=list&sort=description\">Description</a></th><th><a href=\"inventory.php?mode=list&sort=wholesale\">Wholesale</th><th><a href=\"inventory.php?mode=list&sort=list\">List</th><th><a href=\"inventory.php?mode=list&sort=stock\">Stock</th><th><a href=\"inventory.php?mode=list&sort=location\">Location</th><th>Links</th>";
        while ($result=mysql_fetch_array($sql))
        {
            echo "<tr><td>{$result['pn']}</td><td>{$result['description']}</td><td>{$result['wholesale']}</td><td>{$result['list']}</td><td>{$result['stock']}</td><td>{$result['location']}</td><td>[<a href='inventory.php?mode=edit&id={$result['id']}'>Edit</a>]  [<a href='inventory.php?mode=delete&id={$result['id']}'>Delete</a>]  [<a href='orders.php?mode=list_c&id={$result['id']}'>View Orders</a>]</tr>";
        }
        echo "</table></center>";