Unobtrusive ajax link

thepage.html

<html>
<head></head>
<body>
    <p><a id="theajaxlink" href="xhrhandler.php?p=1&q=2">text</a></p>
    <script type="text/javascript">
        (function () {
            'use strict';
            var clickhandler = function (e) {
                var $that = $(this);
                e.preventDefault();
                $.ajax({
                    url: $that.attr('href'),
                    data: {'__from': '__xhr'},
                    success: function () {
                        // whatever
                    }
                });
                return false;
            };
            jQuery(function ($) {
                $('#theajaxlink').on('click', clickhandler);
            });
        }());
    </script>
</body>
</html>

xhrhandler.php

<?php
$ajax = (isset($_GET['__from']) && $_GET['__from'] === '__xhr')
    || (isset($_SERVER['HTTP_X_REQUESTED_WITH'])
        && $_SERVER['HTTP_X_REQUESTED_WITH'] === 'XMLHttpRequest');

// 1. Do whatever work is handler script is supposed to do.

if ($ajax) {
    // 2. Output HTTP body content, if any.
} else {
    // 3. Redirect to 'thepage.html'.
    // If @2. would have had any body output then it MUST be reflected in thepage.html 
    // when the client requests it after redirect @3. <-- is a requirement on thepage.html.
}