nav_component.php

<nav class="navbar navbar-expand-lg navbar-dark bg-primary bg-primary-gradient">
    <a class="navbar-brand d-flex align-items-center fw-500" href="users.html"><img alt="logo"
                                                                                    class="d-inline-block align-top mr-2"
                                                                                    src="img/logo.png"> Учебный
        проект</a>
    <button aria-controls="navbarColor02" aria-expanded="false" aria-label="Toggle navigation" class="navbar-toggler"
            data-target="#navbarColor02" data-toggle="collapse" type="button"><span class="navbar-toggler-icon"></span>
    </button>
    <div class="collapse navbar-collapse" id="navbarColor02">
        <ul class="navbar-nav mr-auto">
            <li class="nav-item">
                <a class="nav-link" href="users.php">Главная <span class="sr-only">(current)</span></a>
            </li>
        </ul>
        <ul class="navbar-nav ml-auto">
            <?php if (is_not_logged_in()): ?>
                <li class="nav-item">
                    <a class="nav-link" href="page_login.php">Войти</a>
                </li>
            <?php else: ?>
                <li class="nav-item">
                    <a class="nav-link" href="logout.php">Выйти</a>
                </li>
            <?php endif; ?>
        </ul>
    </div>
</nav>