<? $nm = "1"; ?>
<form method="post">
<select name="select" onchange="this.form.submit()">
<option value="1" <? if($_POST['select'] == "1"): ?>selected="selected"<? endif ?>>1</option>
<option value="2" <? if($_POST['select'] == "2"): ?>selected="selected"<? endif ?>>2</option>
</select>
</form>
<?
if(isset($_POST['select'])) $nm= $_POST['select'];
echo "Город: ".$nm;
?>