Untitled

<?php

    session_start();
    if($_SERVER['REQUEST_METHOD'] == "POST")
    {
        if($_POST['user'] === 'mysecret' && $_POST['pass'] === 'page')
            $_SESSION['auth'] = true;
    }

    if(isset($_SESSION['auth']))
    {
        ?>
        <html>
        <body>
        <div id="anothercontainer"> </div>
        <div id="container">
        <p id="mycontent"> Here is my new content </p> </div>
        </body>
        </html>
        <?php
        exit();
    }

?>

<html>
    <body>
        <form id="changing" action="test3.php" method="POST">
            User:
            <input type="text" name="user" />
            Pass:
            <input type="password" name="pass" />
            <input type="submit" value="Send" />

        </form>
        <div id="content"> Here is the content to be updated </div>

        <script src="scripts/jquery-3.2.0.js"></script>
        <script>
            $('#changing').on('submit', function(e) {
                    e.preventDefault();
                    var $content = $('#content');
                    var details = $('#changing').serialize();
                    $.post('test3.php', details, function(data) {
                            $content.html( $(data).find('#container') );
                    });
            });
        </script>
    </body>
</html>

success: function(data) {                               // Show content
  $content.html( $(data).find('#container') ).hide().fadeIn(400);
},