When I run this code, I get an error saying that userfile_size is undefined. According to the book I am reading, this variable should automaticly be defined,a nd hold the size of the file uploaded. Am I missing something?

PHP Code:
<html>
<head>
<title>Uploading</title>
</head>
<body>
<h1>Uploading file...</h1>
<?php
if ($userfile=="none")
{
    echo 
"Problem: no file uploaded";
    exit;
}
if (
$userfile_size==0)
{
    echo 
"Problem: uploaded file is zero length";
    exit;
}
if (
$userfile_type != "text/plain")
{
    echo 
"Problem: file is not plain text";
    exit;
}
$upfile "c:\\".$userfile_name;
if (!
copy($userfile$upfile))
{
    echo 
"Problem: Could not move file into directory";
    exit;
}
echo 
"File uploaded successfully<br><br>";

?>
</body></html>