php中实现倒计时的功能

<?phpheader("Content-Type: text/html; charset=utf-8");//防止页面乱码//php的时间是以秒算。js的时间以毫秒算date_default_timezone_set('PRC');//date_default_timezone_set("Asia/Hong_Kong");//地区//配置每天的活动时间段$starttimestr = "2013-12-22 8:10:00";$endtimestr = "2013-12-24 9:43:00";$starttime = strtotime($starttimestr);$endtime = strtotime($endtimestr);$nowtime = time();if ($nowtime<$starttime){die("活动还没开始,活动时间是：{$starttimestr}至{$endtimestr}");}if ($endtime>=$nowtime){$lefttime = $endtime-$nowtime; //实际剩下的时间（秒）}else{$lefttime=0;die("活动已经结束！");}?><script language="JavaScript">var runtimes = 0;function GetRTime(){var nMS = <?php echo $lefttime; ?>*1000-runtimes*1000;if (nMS>=0){var nD=Math.floor(nMS/(1000*60*60*24))%24;var nH=Math.floor(nMS/(1000*60*60))%24;var nM=Math.floor(nMS/(1000*60)) % 60;var nS=Math.floor(nMS/1000) % 60;document.getElementById("RemainD").innerHTML=nD;document.getElementById("RemainH").innerHTML=nH;document.getElementById("RemainM").innerHTML=nM;document.getElementById("RemainS").innerHTML=nS;if(nMS==5*60*1000){alert("还有最后五分钟！");}runtimes++;setTimeout("GetRTime()",1000);}}window.onload=GetRTime;</script><h4>距离活动结束还有 <strong id="RemainD">XX</strong>天 <strong id="RemainH">XX</strong>小时 <strong id="RemainM">XX</strong>分钟 <strong id="RemainS">XX</strong>秒</h4>