PHP+JavaScript 動態下拉式選單select資料庫的值至select

利用onchange+submit 將 select 的值傳給自己，接著select multiple就會更新

if($_POST['class'] != null){
  $class = $_POST['class'];
}
  else{
    $class = '1';
  }

<form name="form" method="post" action="a.php" onSubmit="return check_filed(this)">
項目：<select name="class"onchange="this.form.submit()">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select><br>
</form>
<script>document.form.class.value = '<?=$class?>';</script> //此行為submit後select的預設值會變成剛剛所選

<td>

<select multiple="" size="23" name="Name[]" id="Name" style="width:270" >
<?php
$sql2 = "select * from test WHERE Class LIKE '$class'";  //將值傳給$class後，select multiple的內容就會更新囉!!
$sql = mysql_query($sql2);
while ($rs2 = mysql_fetch_array($sql))
{ 

　echo "<option value='$rs2[Name]'>"; //Name為資料庫欄位名稱
　echo $rs2[Name];
　echo "</option>";
}
?>
</option></select>
</td>