Everything else seems to run and it brings me to the protected.php page of the site. I followed the tutorial pretty closely as well. Here's the code along with a screenshot database.
register.php
<?php
 
include('core/init.inc.php');
 
$errors = array();
 
if (isset($_POST['username'], $_POST['password'], $_POST['repeat_password'])){
        if (empty($_POST['username'])){
                $errors[] = 'The username cannot be empty.';
        }
       
        if (empty($_POST['password']) || empty($_POST['repeat_password'])){
                $errors[] = 'The password cannot be empty.';
        }
       
        if ($_POST['password'] !== $_POST['repeat_password']){
                $errors[] = 'Password verification failed.';
        }
       
        if (user_exists($_POST['username'])){
                $errors[] = 'The username you entered is already taken.';
        }
       
        if (empty($errors)){
                add_user($_POST['username'], $_POST['password']);
               
                $_SESSION['username'] = htmlentities($_POST['username']);
               
                header('Location: protected.php');
                die();
               
        }
               
       
}
 
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
        <!--<link rel="stylesheet" type="text/css" href="ext/css/style.css" />-->
                <title></title>
        </head>
        <body>
                <div>
                        <?php
 
                        if (empty($errors) === false){
                                ?>
                                <ul>
                                        <?php
 
                                        foreach ($errors as $error) {
                                                echo "<li>{$error}</li>";
                                        }
 
                                        ?>
                                </ul>
                                <?php
                        }
 
                        ?>
                </div >
                <form action="" method="post">
                        <p>
                                <label for="username">Username:</label>
                                <input type="text" name="username"  id="username" value="<?php if (isset($_POST['username'])) echo htmlentities($_POST['username']) ?>" />
                        </p>
                        <p>
                                <label for="password">Password:</label>
                                <input type="password" name="password"  id="password" />
                        </p>
                        <p>
                                <label for="repeat_password">Confirm Password:</label>
                                <input type="password" name="repeat_password"  id="repeat_password" />
                        </p>
                        <p>
                                <input type="submit" value="Register" />
                        </p>
                </from>
        </body>
</html>
init.inc.php
<?php
 
session_start();
 
$exceptions = array('register','login');
 
$page = substr(end(explode('/', $_SERVER['SCRIPT_NAME'])), 0, -4);
 
if (in_array($page, $exceptions) === false){
        if (isset($_SESSION['username']) === false){
                header('Location: login.php');
                die();
        }
}
 
mysql_connect("sample_server","roxy503","sample_pass");
mysql_select_db('roxy503');
$path = dirname(__FILE__);
include("{$path}/inc/user.inc.php");
?>
user.inc.php
<?php
 
// Checks to see if the username exists.
function user_exists($user){
        $user = mysql_real_escape_string($user);
 
        $total = mysql_query("SELECT COUNT(`user_id`) FROM `users` WHERE `user_name` = '{$user}'");
 
        return (mysql_result($total, 0) == '1') ? true : false;
}
 echo mysql_error();
// Checks if the Username and Password combination is valid.
function valid_credentials($user, $pass){
        $user = mysql_real_escape_string($user);
        $pass = sha1($pass);
       
        $total = mysql_query("SELECT COUNT(`user_id`) FROM `users` WHERE `user_name` = '{$user}' AND 'user_password' = '{$pass}'");
       
        return (mysql_result($total, 0) == '1') ? true : false;
}
 
//  Adds a user to the database.
function add_user($user, $pass){
        $user = mysql_real_escape_string(htmlentities($user));
        $pass = sha1($pass);
       
        mysql_query("INSERT INTO `users` (`user_password`) VALUES ('{$user}', '{$pass}')");
}
 
?>

