javascript - php用户后台管理不能全部选中？

<code><?php include("../../admin/public/header.php");
include("../../admin/public/admin_left.php");
$sql="select * from ts_users ";
$res=mysql_query($sql);
$arr=array();
if(mysql_num_rows($res)>0){
    while(($list=mysql_fetch_assoc($res))!=false){
        $arr[]=$list;
    }
}
?>
<div id="div">
<table border="0" cellspacing="0" width="900px">
    <tr height="60px">
        <th>选择</th>
        <th>用户名</th>
        <th>邮箱</th>
        <th>手机</th>
        <th>创建事件</th>
        <th>最后登陆时间</th>
        <th>操作</th>
    </tr>
    <?php if(isset($arr)) {
        foreach ($arr as $vo) {
                ?>
                <tr align="center" height="30px">
                    <td id="td"><input type="checkbox" value="<?=$vo['uid']?>"></td>
                    <td>= $vo['uname'] ?></td>
                    <td>= $vo['email'] ?></td>
                    <td>= $vo['phone'] ?></td>
                    <td>= $vo['ctime'] ?></td>
                    <td>= $vo['mtime'] ?></td>
                    <td>
                        <a href="./delete_uid.php?uid=<?=%20%24vo%5B'uid'%5D%20?>">删除</a>
                        <a href="./update_uid.php?uid=<?=%20%24vo%5B'uid'%5D%20?>">编辑</a>
                    </td>
                </tr>
                <?php }
    }
        ?>
    </table>
    <input type="button" value="全选" onclick="allcheck()">
    <input type="button" value="反选" onclick="check()">
    </div>
<script>
    var div=document.getElementById("td");
    var childs=div.childNodes;
    function allcheck(){
        for(var i=0;i<childs.length;i++){
            if(childs[i].nodeName=="INPUT" && childs[i].getAttribute('type')=='checkbox'){
                childs[i].checked=true;
            }
        }
    }
    function check(){
        for(var i=0;i<childs.length;i++){
            if(childs[i].nodeName=="INPUT" && childs[i].getAttribute('type')=='checkbox'){
                if(childs[i].checked==true){
                    childs[i].checked=false;
                }else{
                    childs[i].checked=true;
                }
            }
        }
    }
</script>

</code>