/* in index.php */
<?php
$headerButtonLink = "nextPage.php";
$headerButtonText = "Go to Next Page";
$headerText = "Page Title";
require 'header.php'; ?>
/* in header.php */
<header>
<div class="h1"><?= $headerText; ?></div>
<div class="button" onclick="window.open('HERE I WANT TO PUT A VARIABLE ','_self',false)"><?= $headerButtonText; ?></div>
</header>
<div class="button" onclick="window.open('<?= $headerButtonLink ?>','_self',false)"><?= $headerButtonText; ?></div>