PHP实现倒计时功能

!DOCTYPE html>
html>
head>
 title>PHP实现倒计时功能/title>
/head>
body>
?php
 
 date_default_timezone_set('PRC');

 $starttimestr = date('Y-m-d H:i:s', strtotime('now'));
 $starttimestr= mktime(0,0,0,7,7,2020); 
 //$endtimestr = date('Y-m-d H:i:s', strtotime('+18 hours 1 minutes'));
 $endtimestr = mktime(18,0,0,date('m'),date('d'),date('Y'));
 $starttime = strtotime($starttimestr);
 $endtime  = strtotime($endtimestr);
 $nowtime  = time();
 if ($nowtime  $starttime) {
  exit("考试已经结束了，考试时间是：{$starttimestr}至{$endtimestr}");
 }
 if ($endtime >= $nowtime) {
  $lefttime = $endtime - $nowtime; //实际剩下的时间（秒）
 } else {
  $lefttime = 0;
  exit("考试已经结束！");
 }
?>
p>考试开始时间：?php echo $starttimestr; ?>/p>
p>考试结束时间：?php echo $endtimestr; ?>/p>
p>
 距离考试结束还有：
 span id="RemainD">/span> 天
 span id="RemainH">/span> 小时
 span id="RemainM">/span> 分钟
 span id="RemainS">/span> 秒
/p>
script type="text/javascript">
 var runtimes = 0;
 function GetRTime() {
  var lefttime = ?php echo $lefttime; ?> * 1000 - runtimes * 1000;
  if (lefttime >= 0) {
   var nD = s.floor(lefttime / (1000 * 60 * 60 * 24)) % 24;
   var nH = Math.floor(lefttime / (1000 * 60 * 60)) % 24;
   var nM = Math.floor(lefttime / (1000 * 60)) % 60;
   var nS = Math.floor(lefttime / 1000) % 60;
   document.getElementById("RemainD").innerHTML = nD;
   document.getElementById("RemainH").innerHTML = nH;
   document.getElementById("RemainM").innerHTML = nM;
   document.getElementById("RemainS").innerHTML = nS;
   runtimes++;
   setTimeout("GetRTime()", 1000);
  } else {
   alert('考试结束了！');
   location.reload();
  }
 }
 onload = function() {
  GetRTime();
 }
/script>
/body>
/html>