问如何通过从另一个下拉列表中选择值来填充下拉列表

EN

<?php $sql = "SELECT * FROM country";
$result = mysql_query($sql);
?>
<select id="country" name='country' onchange="get_states();">
<option value=''>Select</option>
<?php while ($row = mysql_fetch_array($result)) {
    echo "<option value='" . $row['country_id'] . "'>" . $row['country_name'] . "</option>";}
?>
</select>
<div id="get_state"></div> // Sub will be appended here using ajax

<script type="text/javascript">
function get_states() { // Call to ajax function
    var country = $('#country').val();
    var dataString = "country="+country;
    $.ajax({
        type: "POST",
        url: "getstates.php", // Name of the php files
        data: dataString,
        success: function(html)
        {
            $("#get_state").html(html);
        }
    });
}
</script>

if ($_POST) {
    $country = $_POST['country'];
    if ($country != '') {
       $sql1 = "SELECT * FROM state WHERE country=" . $country;
       $result1 = mysql_query($sql1);
       echo "<select name='state'>";
       echo "<option value=''>Select</option>"; 
       while ($row = mysql_fetch_array($result1)) {
          echo "<option value='" . $row['state_id'] . "'>" . $row['state_name'] . "</option>";}
       echo "</select>";
    }
    else
    {
        echo  '';
    }
}