Dynamically changing navigation links (next and previous) via AJAX

I found it

First, I want to note that the approach I mentionned in my question is bad according to almost tutorials talking about AJAX in wordpress. So I decided to change the approach and use the built-in AJAX of WordPress.

In other words, the best way for my situation is to use the wp-admin/admin-ajax.php. AJAX requests should be directed to this file. I know the “admin” part of the file name is a bit misleading. but all requests in the front-end (the viewing side) as well as the admin panel can be processed in admin-ajax.php, with a lot of benefits, especially for security.

The steps are:

1.The JavaScript code that submits the AJAX request should look something like this:

  $(document).ready(function() {
        $('.select2').change(function(e) {
    e.preventDefault();

    var v = $('.select2 option:selected').val(); 


            $.ajax({
                type: "GET",
                url: "wp-admin/admin-ajax.php", // check the exact URL for your situation
                dataType: 'html',
                data: ({ action: 'nextPrevious', id: v}),
                success: function(data){

                $('#nav-above').html(data);

                },
                error: function(data)  
                {  
                alert("Your browser broke!");
                return false;
                }  

            }); 

    }); 
}); 

Note that you should respect the requeriements of WordPress in putting the JS script (generally in footer.php before wp-footer() )

2- Handling the action:

in functions.php of your theme (or directly in your plugin file), add:

add_action('wp_ajax_nextPrevious', 'nextPrevious');
add_action('wp_ajax_nopriv_nextPrevious', 'nextPrevious');

and define in the same file nextPrevious callback function like this:

function nextPrevious() {

$p= $_GET['id'];
$my_query = new WP_Query();

$my_query->query(array( 'post__in' => array($p)));

if ($my_query->have_posts()) : while ($my_query->have_posts()) : $my_query->the_post(); ?>

<div class="nav-previous"><?php previous_post_link( '%link', '<img height="34" src="' . get_bloginfo("template_directory") . '/images/previous.png" />' ); ?></div>


                        <div class="nav-next"><?php next_post_link( '%link', '<img height="34" src="' . get_bloginfo("template_directory") . '/images/next.png" />' ); ?></div>

<?php endwhile;
endif;                  

wp_reset_query();

die();

}

Do not forget the die function, it is mandatory (thank you Bainternet).

For more details about AJAX in WordPress, Google first page tutorials are good.