problem with ajax and the path to the php page

i would like to call a php page from an Ajax method : the alert works fine, the code as well has been tested outside of wordpress, but here it seems that the php page is never called.

I found some articles, among them : How to manage ajax calls and JSON in wordpress
but i get more confused. Can you please tell me what should i do here? (with “diapo.php” it doesn’t work, and like that neither)

here is the call with jquery :

        $('.sous-cat').find('img').click(function(){
                var alt_p = $(this).attr('alt');
                //alert(alt_p);

                $.ajax({
                    type: "POST",
                    url: "<?php bloginfo( 'template_directory' ) ?>/diapo.php",
                    data: {p:alt_p},
                    success: function(data) {
                        alert(data);
                    }
                });
                return false;
        });

and the php page for now :

        <?php
        /*
        $p = $_POST['p'];
        query_posts( 'cat=3&p=$p' );
        while (have_posts()) : the_post();
            $result = the_post_thumbnail('normal');
        endwhile;
        wp_reset_query();*/

        echo '$result';

        ?>

Thanks for your help

To use the WordPress ajax url you can pass the var using wp_localize_script:

wp_enqueue_script( 'functions', get_bloginfo( 'stylesheet_directory' ) . '/js/functions.js', array( 'jquery' ), false);
    wp_localize_script( 'functions', 'MyAjax', array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) ) );

In your functions.js

        $.ajax({
                type: "POST",
                url:  MyAjax.ajaxurl,
                data: {p:alt_p},
                success: function(data) {
                    alert(data);
                }
            });