header.php

<?php
if(isset($message)){
   foreach($message as $message){
      echo '
      <div class="message">
         <span>'.$message.'</span>
         <i class="fas fa-times" onclick="this.parentElement.remove();"></i>
      </div>
      ';
   }
}
?>

<nav class="navbar navbar-expand-lg ">
    <div class="container-fluid">
        <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>
        <div class="collapse navbar-collapse" id="navbarNav">
            <ul class="navbar-nav mx-auto">
            <li class="nav-item">
                    <a class="nav-link" href="home">Home</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#about">About</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="contact">Contact</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="shop">Shop</a>
                </li>
              
                <?php if(!isset($_SESSION['user_id'])) { ?>
                    <li class="nav-item">
                        <a class="nav-link" href="login">Login</a>
                    </li>
                <?php } ?>
            </ul>
            <?php if(isset($_SESSION['user_id'])) { ?>
                <div class="d-flex align-items-center">
                    
                    <?php
                        $select_cart_count = mysqli_query($conn, "SELECT * FROM `cart` WHERE user_id = '$user_id'") or die('query failed');
                        $cart_num_rows = mysqli_num_rows($select_cart_count);
                    ?>
                    <a href="cart" class="btn btn-sm btn-outline-light me-2"><i class="fas fa-shopping-cart"></i><span>(<?php echo $cart_num_rows; ?>)</span></a>
                    <div class="dropdown">
                        <button class="btn btn-sm btn-outline-light dropdown-toggle" type="button" id="dropdownMenuButton" data-bs-toggle="dropdown" aria-expanded="false">
                            <?php echo $_SESSION['user_name']; ?>
                        </button>
                        <ul class="dropdown-menu dropdown-menu-end" aria-labelledby="dropdownMenuButton">
                            <li><a class="dropdown-item" href="#"><?php echo $_SESSION['user_email']; ?></a></li>
                            <li><hr class="dropdown-divider"></li>
                            <li>
                             <a class="dropdown-item" href="orders">Orders</a>
                             </li>
                            <li><a class="dropdown-item" href="logout.php">Logout</a></li>
                        </ul>
                    </div>
                </div>
            <?php } ?>
        </div>
    </div>
</nav>