header.php

<nav class="navbar navbar-dark navbar-expand-md navigation-clean-search">
    <div class="container-fluid"><a class="navbar-brand" href="index.php">Photong</a><button data-toggle="collapse" class="navbar-toggler" data-target="#navcol-1"><span class="sr-only">Toggle navigation</span><span class="navbar-toggler-icon"></span></button>
        <div class="collapse navbar-collapse"
             id="navcol-1">
            <ul class="nav navbar-nav">
                <li class="nav-item" role="presentation"><a class="nav-link" href="index.php">Home</a></li>
                <li class="nav-item dropdown"><a class="dropdown-toggle nav-link" data-toggle="dropdown" aria-expanded="false" href="#">About Photong </a>
                    <div class="dropdown-menu" role="menu">
                        <a class="dropdown-item" role="presentation" href="what.php">What is Photong?</a>
                        <a class="dropdown-item" role="presentation" href="how.php">How does Photong work?</a>
                        <a class="dropdown-item" role="presentation" href="who.php">Who created Photong?</a>
                    </div>
                </li>
                <li class="nav-item dropdown"><a class="dropdown-toggle nav-link" data-toggle="dropdown" aria-expanded="false" href="#">Extras </a>
                    <div class="dropdown-menu" role="menu">
                        <a class="dropdown-item" role="presentation" href="credits.php">Credits</a>
                        <a class="dropdown-item" role="presentation" href="tnc.php">Terms and Conditions</a>
                        <a class="dropdown-item" role="presentation" href="privacy.php">Privacy Policy</a>
                    </div>
                </li>
            </ul>
    </div>
</nav>