user_header.php

<header class="navigation fixed-top">
  <nav class="navbar navbar-expand-lg navbar-dark">
   
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navigation"
      aria-controls="navigation" aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse text-center" id="navigation">
      <ul class="navbar-nav ml-auto">
        <li class="nav-item active">
          <a class="nav-link" href="user_home.php">Home</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="product_shop.php">Shop</a>
        </li>
        <li class="nav-item dropdown">
          <a class="nav-link dropdown-toggle" href="#" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">category</a>
          <div class="dropdown-menu">

            <a class="dropdown-item" href="painting.php">painting</a>
            <a class="dropdown-item" href="sketch.php">sketch</a>
            <a class="dropdown-item" href="handicraft.php">Handicraft</a>
          </div>
        <li class="nav-item">
          <a class="nav-link" href="su_chat2.php">Chat</a>
        </li>
       
       
        
        <li class="nav-item">
          <a class="nav-link" href="logout.php">Log out</a>
        </li>
        
       
        
      </ul>
    </div>
  </nav>
</header>