<?php
if($_POST){
$combo = $_POST['combo'];
//simpan data
}
<form name="form1" action="" method="post">
<select name="combo" onchange="document.getElementById('form1').submit()">
<option value="">------</option>
<option value="1">Satu</option>
<option value="2">Dua</option>
</select>
</form>