if (count($errors) == 0) { // Если пользователь ввел свои данные правильно, то продолжаем
$password = md5($password); // дешифровка пароля
$query = "SELECT username, password, email FROM trackowners WHERE username='$username' AND password='$password'"; // запрос данных
$results = mysqli_query($db, $query);
if (mysqli_num_rows($results) == 1) {
  ///////////
  $_SESSION['username'] = $username;
  $_SESSION['success'] = "You are now logged in";
  $_SESSION['email'] = $email;
  header('location: index.php');
}else {
  array_push($errors, "Wrong username/password combination");
} }

<?php  if (isset($_SESSION['username'])) : ?>
    <p>Welcome <strong><?php echo $_SESSION['username']; ?></strong> // Here are your profile settings.</p>

    <p> <a href="index.php?logout='1'" style="color: red;">logout</a> </p>
<?php endif ?>

  <?php  if (isset($_SESSION['username'])) : ?>
  <p>Your e-mail<strong><?php echo $_SESSION['email']; ?></strong> //</p>
<?php endif ?>