JavaScript

$(window).scroll(function () {
    if ($(window).scrollTop() >= $(document).height() - $(window).height() - 10) {
        /***update options***/
        $.ajax({
            type:   \'POST\',
            url:    \'http://www.example.com/wp-admin/admin-ajax.php\',
            data:   {
                action    : \'hit-bottom\',
                option    : $option, // your option variable
                new_value : $new_value // your new value variable
            },
            dataType: \'json\'
        }).done(function( json ) {
            alert( "Ajax call succeeded, let\'s see what the response was." );
            if( json.success ) {
                alert( "Function executed successfully and returned: " + json.message );
            } else if( !json.success ) {
                alert( "Function failed and returned: " + json.message );
            }
        }).fail(function() {
            alert( "The Ajax call itself failed." );
        }).always(function() {
            alert( "This message is always displayed, whether the call failed or succeeded." );
        });
    }
});

functions.php

/* hit bottom of screen event  */
add_action( \'wp_ajax_hit-bottom\', \'wpse_hit_bottom\' );
add_action( \'wp_ajax_nopriv_hit-bottom\', \'wpse_hit_bottom\' );
function wpse_hit_bottom() {
    $option = $_POST[\'option\'];
    $new_value = $_POST[\'new_value\'];

    if( !isset( $option ) || $option == \'\' || !isset( $new_value ) || $new_value = \'\' ) {
        die(
            json_encode(
                array(
                    \'success\' => false,
                    \'message\' => \'Missing required information.\'
                )
            )
        );
    }

    update_option( $option, $new_value );
    die(
        json_encode(
            array(
                \'success\' => true,
                \'message\' => \'Database updated successfully.\'
            )
        )
    );
}