當(dāng)我的計時器計數(shù)為00:00:00 alert('Get Result')按鈕或提交表單時我什至不能在response.php中發(fā)出警報消息。這是我從數(shù)據(jù)庫中選擇持續(xù)時間的地方while($row=mysqli_fetch_array($res)){    $duration=$row["duration"];}$_SESSION["duration"]=$duration;$_SESSION["start_time"]=date("Y-m-d H:i:s");$end_time=$end_time=date('Y-m-d H:i:s', strtotime('+'.$_SESSION["duration"].'minutes',strtotime($_SESSION["start_time"])));$_SESSION["end_time"]=$end_time;這是response.php<?phpsession_start();$from_time1=date('Y-m-d H:i:s');$to_time1=$_SESSION["end_time"];$timefirst=strtotime($from_time1);$timesecond=strtotime($to_time1);$differenceinseconds=$timesecond-$timefirst;if($differenceinseconds<0){    // This is for when timer smaller then 0 then = 00:00:00    $differenceinseconds=0;    echo "TIME UP<br>";//I try to alert a simple message here, and dint work. Why is this happen}echo gmdate("H:i:s",$differenceinseconds);?>這是測驗頁面中的javascript<script type="text/javascript">    var x =setInterval(test,1000);    function test()    {        var xmlhttp=new XMLHttpRequest();        xmlhttp.open("GET","response.php",false);        xmlhttp.send(null);        document.getElementById("response").innerHTML=xmlhttp.responseText;    }</script>顯示計時器標(biāo)簽<div id=response class=timer style=font-size:30px></div>表單名稱和按鈕<form name=myfm method=post action=quizz.php>    <input type=submit name=submit value='Get Result'>