php echo selected option
<select name="s" size="20">
<option value="402" <?php if ($_GET['s'] == '402') { echo 'selected'; } ?>>Audi</option>
<option value="74" <?php if ($_GET['s'] == '74') { echo 'selected'; } ?>>BMW</option>
<option value="201" <?php if ($_GET['s'] == '201') { echo 'selected'; } ?>>Ford</option>
<option value="35" <?php if ($_GET['s'] == '35') { echo 'selected'; } ?>>Saab</option>
</select>