index.html ise index.php olarak değiştirin sonu.php olsun
<!DOCTYPE html>

<html>

<head>

<title></title>

<meta charset="utf-8">

</head>

<body>
<?php $baslik1 = "Deneme Başlık 1" ?>
<h1>

<?php

echo $baslik1;

?>

</h1>



</body>

</html>