// your ajax.php
if (isset($_POST['checkLikes'])) {
exit('hello world');
}

// your test.php
<div id="response"></div>

<script
  src="https://code.jquery.com/jquery-3.3.1.min.js"
  integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
  crossorigin="anonymous"></script>
<script>
    setInterval(function(){
        $.ajax({
            type: "POST",
            url: "ajax.php",
            data: {'checkLikes' : 'test'},
            success: function (response) {
                $('#response').html(response);
                console.log(response);
            }
        })
    });
</script>