Estou aprendendo MySQL e PHP e atualmente estou tendo problemas com esse processo de login.

Parece que a consulta do MySQL não está retornando nenhuma linha e não está me redirecionando após um login bem-sucedido.

PHP:

$error_msg = "";
//checks the session to see if the user is logged in
if (!isset($_SESSION['userid'])) {
    if (!isset($_POST['submit'])) {
        $dbc = mysqli_connect('localhost', 'root', '', 'login')
        or die('Error Connecting to Database on the SQL Server');
        //grabs data from POST
        $user_username = $_POST['username'];
        $user_password = $_POST['password'];
        //lookup username and password in the database
        if (!empty($user_username) && !empty($password)) {
            $query = "SELECT userid, username FROM tuser WHERE username = '$user_username' AND password = SHA('$user_password')";
            $res = mysqli_query($dbc, $query);
            //login is ok so set the user ID and username cookies, redirect to homepage
            if (mysqli_num_rows($res) == 1) {
                $row = mysqli_fetch_array($res);
                $_SESSION['userid'] = $row['userid'];
                $_SESSION['username'] = $row['username'];
                setcookie('userid', $row['userid'], time() + (60 * 60 * 24 * 30), "/");
                setcookie('username', $row['username'], time() + (60 * 60 * 24 * 30), "/");
                setcookie('email', $row['email'], time() + (60 * 60 * 24 * 30), "/");
                setcookie('password', $row['password'], time() + (60 * 60 * 24 * 30), "/");
                //redirect after successful login
                header("Location: index.php");
            } else {
                //the username and password are incorrect so set error message
                $error_msg = 'Sorry, you must enter a valid username and password to log in. <a href="Signup.php">Please sign up!</a>';
            }
        }
    }
}

HTML:

<form action="login_process.php" method="post" class="login">
 Username: <input type="text" name="username"/>
 Password: <input type="password" name="password"/>
 <input type="submit" value="submit"/>
</form>

Desculpe pelo código desleixado, estou familiarizado com PHP e MySQL e me ensinando como fazer isso. Fiz algumas alterações e o código submete, mas ele não me redireciona, ele só vai para login_process.php e não faz nada. O que estou perdendo? Eu aprecio toda a ajuda, obrigado a todos!

    <?
    session_start();

    $error_msg = "";

    //checks the session to see if the user is logged in

if (!isset($_SESSION['user_id'])) 
{
    if (isset($_POST['submit'])) 
    {

        $user_username = mysql_real_escape_string($_POST['username']);
        $user_password = mysql_real_escape_string($_POST['password']);

        $dbc = mysqli_connect('localhost', 'root', '', 'login')
        or die('Error Connecting to Database on the SQL Server');
        //grabs data from POST

            //lookup username and password in the database
            if (!empty($user_username) && !empty($password)) 
            {

                $query = "SELECT userid, username FROM tuser WHERE username = '$user_username' AND password = SHA('$user_password')";

                $res = mysqli_query($dbc, $query);

                //login is ok so set the user ID and username cookies, redirect to homepage
                if (mysqli_num_rows($res) == 1) 
                {
                    $row = mysqli_fetch_array($res);

                    $_SESSION['user_id'] = $row['userid'];
                    $_SESSION['user'] = $row['username'];
                    setcookie('userid', $row['userid'], time() + (60 * 60 * 24 * 30), "/");
                    setcookie('username', $row['username'], time() + (60 * 60 * 24 * 30), "/");
                    setcookie('email', $row['email'], time() + (60 * 60 * 24 * 30), "/");
                    setcookie('password', $row['password'], time() + (60 * 60 * 24 * 30), "/");

                    //redirect after successful login
                    header("Location: home.php");
                    echo mysqli_error($dbc);

                } 
                else 
                {
                    //the username and password are incorrect so set error message
                    $error_msg = 'Sorry, you must enter a valid username and password to log in. <a href="Signup.php">Please sign up!</a>';

                    header("Location: home.php");
                    echo mysqli_error($dbc);

                }
            }
            else
            {
                $error_msg = 'Please enter a username and password to log in. <a href="Signup.php">Please sign up!</a>';

                header("Location: home.php");
                echo mysqli_error($dbc);

            }

    }
}

    ?>