使用NetBeans调试PHP脚本以修改表记录时，'affected_rows'将从1更改为-1

I am testing a simple PHP-MySQL script, and it's to delete one record from the table. The strange thing is in this block of code:

// Check if the form has been submitted:
if ($_SERVER['REQUEST_METHOD'] == 'POST') {

    if ($_POST['sure'] == 'Yes') { // Delete the record.

        // Make the query:
        $q = "DELETE FROM users WHERE user_id=$id LIMIT 1";     
        $r = @mysqli_query ($dbc, $q);
        if (mysqli_affected_rows($dbc) == 1) { // If it ran OK.

When I use NetBeans to debug this script, after the record is deleted($r = @mysqli_query ($dbc, $q) is executed), the affected_rows = 1 in the variable section of NetBeans, which is correct. But then after I press F7 to step into and 'if (mysqli_affected_rows($dbc) == 1)' is executed, affected_rows suddenly becomes -1, and the program logic jumps to the error reporting branch.

If I don't debug and just run the script, the Deletion is totally OK. What's the possible cause?

Here's the whole script:

    <?php # Script 10.2 - delete_user.php
// This page is for deleting a user record.
// This page is accessed through view_users.php.

$page_title = 'Delete a User';
include ('includes/header.html');
echo '<h1>Delete a User</h1>';

// Check for a valid user ID, through GET or POST:
if ( (isset($_GET['id'])) && (is_numeric($_GET['id'])) ) { // From view_users.php
    $id = $_GET['id'];
} elseif ( (isset($_POST['id'])) && (is_numeric($_POST['id'])) ) { // Form submission.
    $id = $_POST['id'];
} else { // No valid ID, kill the script.
    echo '<p class="error">This page has been accessed in error.</p>';
    include ('includes/footer.html'); 
    exit();
}

require ('./mysqli_connect.php');

// Check if the form has been submitted:
if ($_SERVER['REQUEST_METHOD'] == 'POST') {

    if ($_POST['sure'] == 'Yes') { // Delete the record.

        // Make the query:
        $q = "DELETE FROM users WHERE user_id=$id LIMIT 1";     
        $r = @mysqli_query ($dbc, $q);
        if (mysqli_affected_rows($dbc) == 1) { // If it ran OK.

            // Print a message:
            echo '<p>The user has been deleted.</p>';   

        } else { // If the query did not run OK.
            echo '<p class="error">The user could not be deleted due to a system error.</p>'; // Public message.
            echo '<p>' . mysqli_error($dbc) . '<br />Query: ' . $q . '</p>'; // Debugging message.
        }

    } else { // No confirmation of deletion.
        echo '<p>The user has NOT been deleted.</p>';   
    }

} else { // Show the form, to confirm that this user should be deleted.

    // Retrieve the user's information:
    $q = "SELECT CONCAT(last_name, ', ', first_name) FROM users WHERE user_id=$id";
    $r = @mysqli_query ($dbc, $q);

    if (mysqli_num_rows($r) == 1) { // Valid user ID, show the form. (Just 1 result as user_id is PK)

        // Get the user's information:
        $row = mysqli_fetch_array ($r, MYSQLI_NUM);

        // Display the record being deleted:
        echo "<h3>Name: $row[0]</h3>
        Are you sure you want to delete this user?";

        // Create the form:
        echo '<form action="delete_user.php" method="post">
    <input type="radio" name="sure" value="Yes" /> Yes 
    <input type="radio" name="sure" value="No" checked="checked" /> No
    <input type="submit" name="submit" value="Submit" />
    <input type="hidden" name="id" value="' . $id . '" />
    </form>';

    } else { // Not a valid user ID.
        echo '<p class="error">This page has been accessed in error.</p>';
    }

} // End of the main submission conditional.

mysqli_close($dbc);

include ('includes/footer.html');
?>

Another problem is that after running the script, there are many lines of warnings:

 Warning: main(): Couldn't fetch mysqli in C:\xampp\htdocs\phpmysql4_working\delete_user.php on line 75

 Warning: main(): Couldn't fetch mysqli in C:\xampp\htdocs\phpmysql4_working\includes\footer.html on line 11
Call Stack
#   Time    Memory  Function    Location
1   0.1000  146128  {main}( )   ..\delete_user.php:0
2   249.5054    187032  include( 'C:\xampp\htdocs\phpmysql4_working\includes\footer.html' ) ..\delete_user.php:75

But MySQL was actually been accessed successfully. The footer.html is:

    <!-- End of the page-specific content. --></div>

    <div id="footer">
        <p>Copyright &copy; <a href="#">Plain and Simple</a> 2007 | 
            Designed by <a href="http://www.edg3.co.uk/">edg3.co.uk</a> | 
            Sponsored by <a href="http://www.opendesigns.org/">Open Designs</a> | 
            Valid <a href="http://jigsaw.w3.org/css-validator/">CSS</a> &amp; <a href="http://validator.w3.org/">XHTML</a></p>
    </div>
</body>
</html>

I investigated, and found that this is not something that Xdebug does wrong, but the MySQLi extension itself. I filed a bug report for PHP at https://bugs.php.net/bug.php?id=67348