Try this:
<select name="Month" size=1>
<?php
$months = array("Dec","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov");
$thisMonth = date("n", mktime());
for ($i=0; $i < 12; $i++) {
printf("<option value=\"%s\">%s</option>", ($thisMonth+$i)%12, $months[($thisMonth+$i)%12]);
}
?>
</select>