js倒计时跳转页面

<div class="success_msg">
恭喜您，数据提交成功，正在处理中，<span id="second" >3</span>秒钟后跳转！
</div>

<script type="text/javascript">

//倒计时
setInterval("clock()", 1000);
function clock() {
var span = document.getElementById('second');
var num = span.innerHTML;
if(num != 0) {
num--;
span.innerHTML = num;
}
};
</script>

<meta http-equiv="refresh" content="3; url=<?php echo $_SERVER['HTTP_REFERER'];?>">