I can not program in PHP

I already have a web application in java and now I want to migrate to php, the problem is that when trying to execute the php code does not show it how it should, I used a test code:

        <button onclick="enviaPeticion()">
        Pedir
    </button>
    <h1 id="titulo"></h1>
    <script>
    function enviaPeticion(){
        var elObjeto= new XMLHttpRequest();
        elObjeto.open('GET','prueba.php', true);

        elObjeto.onreadystatechange = function(){
           document.getElementById('titulo').innerHTML = elObjeto.responseText;

        }
        elObjeto.send();
    }
    </script>

In the php file it is:

<?php
echo 'Hola Mundo';
?>

When trying to execute it through AJAX he sends me the following answer:

<!--?php
echo 'Hola Mundo';
?-->

In html tags I already know that

<!-- -->

is to comment, what I do not understand is why he sends it that way, if someone could help me, I would be very grateful