<?php
echo <<<DATA

qui va il codice html...

DATA;
?>

<?php

include('header.php');
include('main.php');
include('footer.php');

?>

<?php

$pagina = ( isset($_GET['pagina']) ) ? $_GET['pagina'].'.php' : 'main.php';

include('header.php');
include $pagina;
include('footer.php');

?>