nav.php

<nav class="navbar navbar-expand-lg navbar-light bg-light">
    <a class="navbar-brand" href="#">CSEC</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarTogglerDemo02" aria-controls="navbarTogglerDemo02" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse" id="navbarTogglerDemo02">
        <ul class="navbar-nav mr-auto mt-2 mt-lg-0">
            <li class="nav-item active">
                <a class="nav-link" href="index.php">Home <span class="sr-only">(current)</span></a>
            </li>
        </ul>
        <a href="login.php"><button type="button" class="btn btn-primary">Login</button></a>
        <a href="registration.php"><button type="button" class="btn btn-success" style="margin-left: 10px;">Register</button></a>
    </div>
</nav>