PHP和js实时倒计时

<?php
//这是t.php页面
header('content-type:text/html;charset=utf-8');
date_default_timezone_set('PRC');
$start_time = '09:00:00';
$end_time = '18:00:00';
$start_famate_time = strtotime($start_time);        //开始时间转化为时间戳
$end_famate_time = strtotime($end_time);          //结束时间转化为时间戳
$now_time = time();
if($end_famate_time < $now_time || $start_time > $now_time){
    echo "当前不在考试的时间范围内！";
exit;
}
$remain_time = $end_famate_time-$now_time;            //剩余的秒数
$remain_hour = floor($remain_time/(60*60));             //剩余的小时
$remain_minute = floor(($remain_time - $remain_hour*60*60)/60); //剩余的分钟数
$remain_second = ($remain_time - $remain_hour*60*60 - $remain_minute*60); //剩余的秒数
echo "距离秒杀结束还有".$remain_hour."小时".$remain_minute."分钟".$remain_second."秒";
?>

<div id="showTime"></div>
<script src='jquery.js'></script>
<script>
    $(function(){
        st();
    });
    var aj;
    function st(){
        aj=$.ajax({
            'url':'t.php',
            'success':function (data){
                $('#showTime').html(data);
                aj.abort();//销毁上一次ajax对象
                st();
            }
        });
    }
</script>