php写web上传程序的一些问题？

错在哪里了阿：客户端：
<form action="file-upload.php" method="POST" enctype="multipart/form-data">
  Send these files:<br>
  <input name="userfile[]" type="file"><br>
  <input type="submit" value="Send files">
</form>file-upload.php文件如下：<html>
<head>
</head>
<body><?php$uploaddir='uploads';
$uploadfile=$uploaddir.$_FILES['userfile']['name'];print "<pre>";
if(move_uploaded_file($_FILES['userfile']['tmp_name'],$uploaddir.$_FILES['userfile']['name'])){ print "File is valid,and was successfully uploaded.Here's some more debugging info:\n";
print_r($_FILES);
}else{ print "Possible file upload attack!Here's some debugging info:\n";
print_r($_FILES);
}print "</pre>";?></body>
</html>想上传一个project.txt文件，结果没有上传成功，出现了如下信息：Possible file upload attack!Here's some debugging info:
Array
(
    [userfile] => Array
        (
            [name] => Array
                (
                    [0] => project.txt
                )            [type] => Array
                (
                    [0] => text/plain
                )            [tmp_name] => Array
                (
                    [0] => /var/tmp/phpD9KkuC
                )            [size] => Array
                (
                    [0] => 434
                )        ))不知道怎么改动，才能把文件上传过去？