I just don't understand what's happening, I haven't change anything to the site for a few months but now suddenly the rss feed doesn't work anymore.

I create a php file with the following code:

header('Content-type: text/xml'); 
include("config/config.inc.php");

    $result = mysqli_query($link, "SELECT * FROM tutorials ORDER BY tutorial_id DESC LIMIT 50");
?>
<rss version="2.0">
    <channel>
         <title>CMS tutorial site</title>
         <description>Bringing you the best CMS tutorials from the web</description>
         <link>http://cmstutorials.org</link>
        <?php 
        while($row = mysqli_fetch_object($result))
        {
            $user = mysqli_fetch_object(mysqli_query($link, "SELECT * FROM user_extra WHERE userid=".$row->user_id.""));
            ?>
            <item>
                <title><?php echo $row->title; ?></title>
                <author><?php echo $user->username; ?></author>
                <description><?php echo $row->description; ?></description>
                <pubDate><?php echo $row->date; ?></pubDate>
                <link>http://cmstutorials.org/view_tutorial.php?tutorial_id=<?php echo $row->tutorial_id; ?></link>
            </item>
            <?php
        }
        ?>
    </channel>
</rss>

I checked the query by executing it in phpmyadmin and it works, doesn't give any error. When I delete the header content type and the rss tag it will print out each line from the query but the feed won't display anything

this is the link of the feed http://cmstutorials.org/rss (or http://cmstutorials.org/rss.php)