Undefined index, trying to get email from DB,

Question

Okay, I cant undestand how can I get the EMail from my DB. I want to display a profile page, where users can change their info, so far I created this: This is the entire profile.php page. Is there a tutorial.

<div class="text-center">
    <h1>Profile Page</h1>
    <hr>
    <br>
</div>

<div class="content">
    <!-- notification message -->
    <?php if (isset($_SESSION['success'])) : ?>
        <div class="error success">
            <h3>
                <?php
                echo $_SESSION['success'];
                unset($_SESSION['success']);
                ?>
            </h3>
        </div>
    <?php endif ?>

    <!-- logged in user information -->
    <?php if (isset($_SESSION['username'])) : ?>
        <div class="container">
            <div class="row text-center">
                <div class="col">
                <h4>Welcome back <strong><?php echo $_SESSION['username']; ?></strong>!</h4>
                <hr>
                <p><strong>Your account details</strong></p>
                <p>Your name is <?php  echo $_SESSION['username']; ?></p>
                <p>Your name is <p>Mail: <?php echo $_SESSION['email'];?></p>
                <p> <a href="logout.php" class="btn btn-danger">Logout</a> </p>
                </div>
                <div class="col text-center">
                    <h4>My Orders [Coming Soon]</h4>
                </div>
            </div>
        </div>
    <?php endif ?>
</div>

</body>

</html>

I tried adding <p>Your name is <p>Mail: <?php echo $_SESSION['email'];?></p> but I still get the following error Undefined index: email, I really want to learn and understand this problem. Thanks