上传文件时PHP中未定义的索引错误

I have such a form for uploading files and creating new directories:

<form  action="files.php"  method="POST">
<div class="row btn-row">
     <div class="col-xs-3">
          <input type="text" class="form-control" id="mkdir-name" placeholder="New directory name" size="10">
     </div>

     <input type="submit" class="btn btn-success my-btn" value="Create folder" name="mkdir">
     <input type="submit" class="btn btn-danger my-btn" value="Delete folder" name="remdir">
     </div>
<div class="row btn-row">
     <div class="col-xs-3">
          <input type="hidden" name="MAX_FILE_SIZE" value="1024000">
          <input name="userfile" type="file" class="my-btn">
     </div>
     <input type="submit" class="btn btn-success my-btn" value="Upload file" name="upfile">
     <input type="submit" class="btn btn-danger my-btn" value="Delete file" name="delfile">
</div>
</div>

</form>

The corresponding files.php script is quite simple:

    <?php
//Start new or resume existing session
    session_start();
    if(isset($_POST['upfile'])){
        $target_dir = $_SESSION["current-directory"]."/";
        $target_file = $target_dir . basename($_FILES["userfile"]["name"]);
        if (move_uploaded_file($_FILES['userfile']['tmp_name'], $target_file)) {
            echo "File is valid, and was successfully uploaded.
";
        } else {
            echo "Possible file upload attack!
";
        }
    }
?>

I'm really new in PHP but I'm completly following different tutorials from w3school and php.net. I'm getting the following errors:

Notice: Undefined index: userfile in C:\wamp\www\webspace\php\files.php on line 6 Notice: Undefined index: userfile in C:\wamp\www\webspace\php\files.php on line 7

I have no idea what can cause this problem.

Use form multi-part inside form tag like this:

<form  action="files.php"  method="POST" enctype="multipart/form-data">