jquery跨域请求json数据

//服务端生成json数据json.php
<?php
$json=array("Volvo","BMW","SAAB");
$cb = $_GET['callback'];
echo $cb.'('.json_encode($json, true).')';
?>

//客户端Ajax请求数据
<script>
$(document).ready(function() {
var url="http://域名/json.php?callback=?"; 
$.getJSON(url,function(data){ 
alert(data);
});
</script>