<?php
// use switch for prevent sql injection
switch ($_GET['sort']){ 
	case 'asc':
        $order = 'ORDER BY `Id` ASC';
	break;

	case 'desc':
        $order = 'ORDER BY `Id` DESC';
	break;
}

$query = "SELECT * FROM `table`" . $order;
// to do it you must use mysql_query()
?>

<label for="list">order by:</label>
    <select onchange="location = '?sort=' + this.options[this.selectedIndex].value;" name="list">
        <option selected value="asc">choose...</option>
        <option  value="asc">ASC</option>
        <option  value="desc">DESC</option>
    </select>
    
<?php
	echo $query;
?>