php文件下传有关问题

php文件上传问题?
html页面代码:

HTML code

<!--

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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>文件上传</title>


<h1>文件上传</h1><br>

选择上传文件

php页面代码

PHP code

<!--

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--><meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<?php if($_FILES['userfile']['error']>0)
    echo "上传失败!!";
else 
    echo "上传成功!!";
if($_FILES['userfile']['type'] !='text/plain')
    echo "请选择正确的文件类型!!";
?>

出现的问题:
Notice: Undefined index: userfile in G:\apache\htdocs\php\upload.php on line 4
上传成功!!
Notice: Undefined index: userfile in G:\apache\htdocs\php\upload.php on line 8
请选择正确的文件类型!!

这是怎么回事呢?照着书上敲的,上传的文件无论就算是text格式的也不能正确判断,是不是处在$_files上面呢?弄了好久没弄明白,希望可以有高人帮助下,谢谢了....


------解决方案--------------------
print_r($_FILES); //看看