下面如何设置 if isset 语句

我在下面有一个javascript函数，当文件完成上传时，它将显示必要的消息：

function stopImageUpload(success){
      var imagename = <?php echo json_encode($imagename); ?>;   
      var result = '';
      if (success == 1){
         result = '<span class="msg">The file was uploaded successfully!</span><br/><br/>';
         $('.listImage').append(imagename + '<br/>');
      }
      else {
         result = '<span class="emsg">There was an error during file upload!</span><br/><br/>';
      }
return true;
}

现在我正在尝试检索下面的 $_SESSION 变量，然后使用上面的 javascript 函数（在单独的页面上）将 $_SESSION 调用为"$imagename"，然后将其附加到 .list 中。

但我的问题是我如何编写上面的 if issett 语句，以便它可以判断是否设置了下面的 $_SESSION 变量，以便我可以使用$imagename来检索会话？

下面是 php 脚本，其中包含 $_SESSION 和上传文件的位置：

    if (isset($_POST['fileImage'])) {
      $_SESSION['imagename'] =  $_FILES['fileImage']['name'];
    if( file_exists("ImageFiles/".$_FILES['fileImage']['name'])) {  
        $_SESSION['fileImage']['name'] = $_FILES['fileImage']['name'];
        $parts = explode(".",$_FILES['fileImage']['name']);
        $ext = array_pop($parts);
        $base = implode(".",$parts);
        $n = 2;
        while( file_exists("ImageFiles/".$base."_".$n.".".$ext)) $n++;
        $_FILES['fileImage']['name'] = $base."_".$n.".".$ext;
        move_uploaded_file($_FILES["fileImage"]["tmp_name"], 
        "ImageFiles/" . $_FILES["fileImage"]["name"]);
        $result = 1;

    }
        else
          {
          move_uploaded_file($_FILES["fileImage"]["tmp_name"],
          "ImageFiles/" . $_FILES["fileImage"]["name"]);
          $result = 1;
          }
      }

    ?>
    <script language="javascript" type="text/javascript">
window.top.stopImageUpload(<?php echo "'$result'";?>);</script>