Warning: Undefined array key "username" in /usr/home/Liga/domains/liga.3d.tc/public_html/navbar.php on line 7

<div class="navbar">
    <div>
        <a href="profile.php?id=0">Profile</a>
        <a href="feed.php">Posts</a>
        <a href="messages.php">Messages</a>
        <a href="settings.php">Settings</a>
      <a href="notifications.php">Notifications
                
    </a>
      <form action="search.php" method="GET">
        <input type="text" name="q" placeholder="Поиск пользователей...">
        <button type="submit">🔍</button>
    </form>
    </div>
    <div>
        <a href="logout.php" class="logout-btn">Logout</a>
    </div>
</div>

<style>
    .navbar {
        background: #007bff;
        padding: 15px;
        display: flex;
        justify-content: space-between;
        align-items: center;
        color: white;
    }
    .navbar a {
        color: white;
        text-decoration: none;
        font-size: 18px;
        margin: 0 10px;
    }
    .logout-btn {
        background: #dc3545;
        color: white;
        padding: 10px;
        border-radius: 5px;
    }
</style>