<?php
if(isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
    header('Content-type: application/json');
    exit(json_encode(array('status' => TRUE, 'data' => nl2br(file_get_contents('msg.txt')))));
}
?>
<!DOCTYPE html>
<html>
<head>
    <title></title>
</head>
<body>
    <div id="result"></div>
    <script src="https://code.jquery.com/jquery-1.11.3.js"></script>
    <script type="text/javascript">
        $(function(){
            let message = function() {
                $.get("", function(json) {
                    if (json.status) {
                        $("#result").html(json.data);
                    }
                
                    setTimeout(function(){
                        message();
                    }, 3000);
                });
            }

            message();
        });
    </script>
</body>
</html>
Test etmedim, basit bir örnek vermek istedim.