index.php

<!doctype html>
<html>
<body>
    <form action="/" method="POST">
        Post Comment: <br>
        <input type="text" style="width: 300px;" name="comment">
        <input type="submit">
    </form>
    <br>


<?php
    if(isset($_POST["comment"]))
    {
        $comment = $_POST["comment"];

        if(file_exists("comments.txt"))
        {
            $comments = file_get_contents("comments.txt");
            $comments = $comments . $comment;
            file_put_contents("comments.txt", $comments);
            echo file_get_contents("comments.txt");
        }
        else
        {
            file_put_contents("comments.txt", $comment);
            echo file_get_contents("comments.txt");
        }
    }
?>

</body>
</html>