<?php

$get = isset($_GET['pag'])? $_GET['pag']:'';

//header, cabeçalho
require 'staff/funcao.php';

require 'pag/header.php';
require 'pag/cabecalho.php';

?>

<?php
//paginas
caminho($get);
?>

<?php
//rodape
require 'pag/rodape.php';
?>

<header class="cabecalho">
<nav class="navbar fixed-top navbar-light bg-light">
<a class="navbar-brand" href="index.php">
  <img src="img/logosemnome.png" width="80px" height="30px" alt="Our lady Agencia digital">
</a>

<ul class="nav justify-content-end">
<li class="nav-item">
  <a class="nav-link active" href="index.php">Inicio</a>
</li>
<li class="nav-item">
  <a class="nav-link" href="#servico">Serviços</a>
</li>
<li class="nav-item">
  <a class="nav-link" href="#criacoes">Criações</a>
</li>
<li class="nav-item">
  <a class="nav-link" href="#oferecer">Oferecemos a você</a>
</li>
<li class="nav-item">
  <a class="nav-link" href="?pag=contato">Contato</a>
</li>
</ul>
</nav>
  </header>

<?php

function caminho($pagina){
switch ($pagina) {
    case 'contato';
        require 'pag/contato.php';
        break;
    default:
        require 'pag/home.php';
        break;
}
}

<header class="cabecalho">
<nav class="navbar fixed-top navbar-light bg-light">
<a class="navbar-brand" href="index.php">
  <img src="img/logosemnome.png" width="80px" height="30px" alt="Our lady Agencia digital">
</a>

<ul class="nav justify-content-end">
<li class="nav-item">
  <a class="nav-link active" href="index.php">Inicio</a>
</li>
<li class="nav-item">
  <a class="nav-link" href="?pag=contato">Contato</a>
</li>
</ul>
</nav>
</header>