ajax jsonP 解决跨域问题

<!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>无标题文档</title>
<script>
    function fn1(data){
        var oUl1 = document.getElementById('ul1');
        var html = '';
        for (var i=0; i<data.length; i++) {
            html += '<li>'+data[i]+'</li>';
        }
        oUl1.innerHTML = html;
    }
    
    function fn2(data){
        var oUl2 = document.getElementById('ul2');
        var html = '';
        for (var i=0; i<data.length; i++) {
            html += '<li>'+data[i]+'</li>';
        }
        oUl2.innerHTML = html;
    }
    window.onload = function(){
        var oBut1 = document.getElementById('btn1');
        var oBut2 = document.getElementById('btn2');
        oBut1.onclick = function(){
            var oScript = document.createElement('script');
            oScript.src = 'jsonP.php?callback=fn1';
            document.body.appendChild(oScript);
        };
        
        oBut2.onclick = function(){
            var oScript = document.createElement('script');
            oScript.src = 'jsonP.php?callback=fn2';
            document.body.appendChild(oScript);
        };
    };
</script>

</head>

<body>
    <input type="button" id="btn1" value="加载数字"/>
    <ul id="ul1"></ul>
    <input type="button" id="btn2" value="加载字母"/>
    <ul id="ul2"></ul>
</body>
</html>

<?php
//$t = isset($_GET['t']) ? $_GET['t']:'num';
$t = isset($_GET['t']) ? $_GET['t'] : 'num';
$callback = isset($_GET['callback']) ? $_GET['callback'] : 'fn1';

$arr1 = array('111111','22222222','33333333','4444444','555555555555555555555');
$arr2 = array('aaaaaaaaaaaa','bbbbbbbb','cccccccccccc','ddddddddd','eeeeeeeeeeee');

if ($callback == 'fn1') {
    $data = json_encode($arr1);
} else {
    $data = json_encode($arr2);
}

echo $callback.'('.$data.');';