php-PHP上传文件的问题$_FILES['file']['erro

这段时间一直在啃《Head First PHP & MySQL》，这本书非常好，一步步教授怎么开发一个
PHP网站，一直照着一步步地敲代码，现在看到487页，在测试上传文件功能时，发现前面
的editprofile.php里面的 if ($_FILES['file']['error'] == 0) { 老是报错：
// Validate and move the uploaded picture file, if necessary
if (!empty($new_picture)) {
if ((($new_picture_type == 'image/gif') || ($new_picture_type == 'image/jpeg') || ($new_picture_type == 'image/pjpeg') ||
($new_picture_type == 'image/png')) && ($new_picture_size > 0) && ($new_picture_size <= MM_MAXFILESIZE) &&
($new_picture_width <= MM_MAXIMGWIDTH) && ($new_picture_height <= MM_MAXIMGHEIGHT)) {
if ($_FILES['file']['error'] == 0) {
// Move the file to the target upload folder
$target = MM_UPLOADPATH . basename($new_picture);
if (move_uploaded_file($_FILES['new_picture']['tmp_name'], $target)) {
// The new picture file move was successful, now make sure any old picture is deleted
if (!empty($old_picture) && ($old_picture != $new_picture)) {
@unlink(MM_UPLOADPATH . $old_picture);
}
}
else {
// The new picture file move failed, so delete the temporary file and set the error flag
@unlink($_FILES['new_picture']['tmp_name']);
$error = true;
echo '

Sorry, there was a problem uploading your picture.

Your picture must be a GIF, JPEG, or PNG image file no greater than ' . (MM_MAXFILESIZE / 1024) .
' KB and ' . MM_MAXIMGWIDTH . 'x' . MM_MAXIMGHEIGHT . ' pixels in size.

// Update the profile data in the databaseif (!$error) {  if (!empty($first_name) && !empty($last_name) && !empty($gender) && !empty($birthdate) && !empty($city) && !empty($state)) {    // Only set the picture column if there is a new picture    if (!empty($new_picture)) {      $query = "UPDATE mismatch_user SET first_name = '$first_name', last_name = '$last_name', gender = '$gender', " .        " birthdate = '$birthdate', city = '$city', state = '$state', picture = '$new_picture' WHERE user_id = '" . $_SESSION['user_id'] . "'";    }    else {      $query = "UPDATE mismatch_user SET first_name = '$first_name', last_name = '$last_name', gender = '$gender', " .        " birthdate = '$birthdate', city = '$city', state = '$state' WHERE user_id = '" . $_SESSION['user_id'] . "'";    }    mysqli_query($dbc, $query);    // Confirm success with the user    echo '
Your profile has been successfully updated. Would you like to view your profile?
';    mysqli_close($dbc);    exit();  }  else {    echo '
You must enter all of the profile data (the picture is optional).
';  }}

修改登录用户的信息并点击按钮时

就会显示下图的错误信息，而且数据库里面的信息根本没有改变！！！！Notice: Undefined index: file in D:\phpStudy\WWW\Mismatch\editprofile.php on line 44第44行是： if ($_FILES['file']['error'] == 0) { 请问这是什么原因，怎么解决？？代码完全是原书官网提供的，没有写错