<nav>
    <ul id="menu">
        <li><a href="index.php?menu=1">O Nas</a></li>
        <li><a href="index.php?menu=2">Oferta</a></li>
        <li><a href="index.php?menu=3">Galeria</a></li>
        <li class="submenu"><a href="index.php?menu=4">Opieka nad grobami</a>
            <ul>
                <li><a href="index.php?menu=4">Oferta</a></li>
                <li><a href="index.php?menu=6">Kontakt</a></li>
            </ul>
        </li>
        <li><a href="index.php?menu=5">Kontakt</a></li>
    </ul>
</nav>
<section>
    <?php
        $arg = (int)$_GET['menu'];

        switch ($arg)
        {
            case 1:
                include('domowa.php');
            break;

            case 2:
                include('oferta.php');
            break;

            case 3:
                include('galeria.php');
            break;

            case 4:
                include('oferta2.php');
            break;

            case 5:
                include('kontakt.php');
            break;

            case 6:
                include('kontakt2.php');
            break;

            default:
                include('domowa.php');
            break;
        }
    ?>
</section>