OK, takže 3...2...1....vypínám mozek....... píšu syntakticky funkční ukázku:

<?
if (!isset($_POST['mnau']))
{
?>
<form action="" method="post">
<select name="mnau" onchange="this.form.submit();">
<option value="jedna">jednicka</option>
<option value="dva">dvojka</option>
<option value="tri">trojka</option>
</select>
</form>
<?
}
else
{
echo '<form action="" method="post">
<select name="mnau" onchange="this.form.submit();">';

foreach ( $_POST as $postitem ) {

switch ($postitem) {
case "jedna":
    echo '<option value="jedna" selected>jednicka</option>
<option value="dva">dvojka</option>
<option value="tri">trojka</option>';
    break;
case "dva":
echo '<option value="jedna">jednicka</option>
<option value="dva" selected>dvojka</option>
<option value="tri">trojka</option>';
        break;
case "tri":
    echo '<option value="jedna">jednicka</option>
<option value="dva">dvojka</option>
<option value="trojka" selected>trojka</option>';
    break;
default:
;
}
}
echo '</select>
</form>';
}
?>