实用小例：
ajaxTest.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title> jquery + ajax + php + select </title>
</head>
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<script language="javascript">
$(document).ready(function(){
$("#bTrade").change(function() { //jquery 中change()函数
$("#quote").load("ajaxTest.php?name="+$("#bTrade").val()); //jqueryajax中load()函数
});
});
</script>
<select name="bTrade" id="bTrade">
<option value="a">a</option>
<option value="b">b</option>
</select>
<div id="quote">
</div>
<body>
</body>
</html>
ajaxTest.php
<?php
$array = array(
'a'=>array('a1','a2','a3','a4'),
'b'=>array('b1','b2','b3','b4'),
);
$name=$_GET['name'];
$str='<select name="server" >';
foreach($array[$name] as $key=>$value)
{
$str.='<option value="'.$key.'">'.$value.'</option>';
}
$str.='</select>';
echo $str;
?>