<?php
$path = $_SERVER['DOCUMENT_ROOT'];
?>
<!DOCTYPE html>
<html lang="en">
<?include "$path/private/head.php"; ?>
<body>
<div class="cont">
<?include "$path/private/header.php"; ?>
<main class="startPage">
<div id="startLogin" class="startbtn">Log In</div>
<div id="startSignup" class="startbtn">Sign Up</div>
</main>
<?include "$path/private/footer.php"; ?>
</div>
</body>
</html>
это уже другой фаил
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<link rel="stylesheet" type="text/css" href='/css/styles.css'>
<script src="/js/jquery-3.7.1.min.js"></script>
<script src="/js/script.js"></script>
</head>