使用ajax覆盖会话变量

我创建了一个YouTube搜索引擎，可以使用会话发送视频id、标题等。我为每个按钮创建了具有唯一id的按钮，通过ajax调用哪个页面，然后使用该按钮的唯一id生成会话。

javascript代码如下：

    <script type="text/javascript">
        function loadXMLSession(videoid, videotitle) {
            var xmlhttp;
            if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
                xmlhttp = new XMLHttpRequest();
            } else {// code for IE6, IE5
                xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            xmlhttp.onreadystatechange = function() {
                if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                    document.getElementById("myDiv").innerHTML = xmlhttp.responseText;
                }
            }
            xmlhttp.open("GET", "GenerateDownloadSession.php?videoid=" + videoid + "&videotitle=" + videotitle, true);
            xmlhttp.send();

            //strip off spaces and embed dashes - clean urls
            var downloadtitle = videotitle;
            downloadtitle = downloadtitle.toLowerCase();
            downloadtitle = downloadtitle.replace("-"," ");//strip off dashes with spaces
            downloadtitle = downloadtitle.replace(/ +(?= )/g,'');//replace multiple spaces with one space
            downloadtitle = downloadtitle.replace(/[`~!@#$%^&*()_|+'-=?;:'",.<>'{'}'[']'''/]/gi, '');//strip off all special characters from video title
            downloadtitle = downloadtitle.replace(/ /g,"-");//replace spaces with dashes
            downloadtitle = downloadtitle.replace(/-+$|(-)+/g, '$1');//replace multiple dashes with single dash
            var url = window.location.hostname;
            url = "/development"//only for development phase
            url = url+"/download/"+downloadtitle+".html";
            window.location=url;
        }
    </script>

下载按钮编码如下：

echo "<button id='"downloadbtn'" class='"btn'" value = '"" . $YouTubeVideoID . "'" onclick='"loadXMLSession(this.value,'" . $VideoContent['6'] . "')'"><img src='"" . $HostURLRedirect . "/img/Arrow-Down-icon.png'" alt='"download'" />&nbsp;Download</button>&nbsp;";

ajax调用的php页面具有简单的会话创建：

session_start();
$videoid = $_GET['videoid'];
$videotitle = $_GET['videotitle'];
$_SESSION['DownloadID'] = $videoid;
$_SESSION['DownloadTitle'] = $videotitle;
$_SESSION['DownloadType'] = "Download";

我遇到的问题是，当我打开浏览器后第一次点击下载按钮时，它运行良好。但当我再次搜索时，它会返回以前的会话值。我通过ajax调用会话函数并将值传递给它。它应该覆盖会话值。但在这种情况下，它并没有覆盖这些值。我该如何克服这个问题？有什么建议吗？