PHP Error - What does my error I am getting mean? - php

Question

I am getting this error after clicking the delete link on the view_users.php page. I think the error is on the delete_users.php page but when I use the data files version that you can download from his site I still get the same error.
This is the error:
Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in /var/www/PHP and MySQL for Dynamic Web Sites/Chapter 9/delete_user.php on line 38
<?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.
// 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.
// 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');
?>

Answer 1

// mysqli_query failed and returned false instead of returning a mysqli_result
$r = #mysqli_query ($dbc, $q);
//on the line below you pass $r which is false because the query failed
if (mysqli_num_rows($r) == 1) {
You should check to see if $r is false and figure out what went wrong
if($r===false)
{
echo mysqli_error($dbc) ;
}

Answer 2

Try checking if there's anything wrong with your database connection. So see if $dbc is throwing any errors, it may be causing mysqli_query()some problems. For debugging sakes remove the error control operator in front of mysqli_query().
Also, see if the query that you're trying to run (SELECT CONCAT(last_name, ', ', first_name) FROM users WHERE user_id=$id) throws any errors. As a quick tip, don't use string interpolation for your queries, try to bind the variables to a prepared statement as parameters.