Qual é a melhor forma de integrar meu php ao html?

Exemplo 1:

Script php dentro do html:

index.php

<?php

    echo "meu script";

?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

</body>
</html>

Exemplo 2:

Chamando o php externo no html:

index.php

<?php

    require 'teste.php';

?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

</body>
</html>

Exemplo 3:

Chamando o php por ajax no js:

index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

    <p id="txt"></p>

</body>
<script>
var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("txt").innerHTML = xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET", "teste.php?", true);
    xmlhttp.send();
</script>
</html>

Qual é melhor meio?

Lembrando que gostaria de dividir as coisa tipo html só com html e php só com php.