无法实施长轮询

I am newbie about long poll. I want to update my wall if any new data in database by long. So as a small field I just try this below script which display undefined continuously in index.php page.

In I used this code in server.php on test, it works well.

while (true) {
    $random = rand(1, 10); 
    $data = ($random == 1) ? array('aaa' => time()) : array();
    if (!empty($data)) {
        echo json_encode($data);
        flush();
        exit(0);
    }
    sleep(5);
}

Here, my problem is occurred when I was going to implement my php query. I have no idea about, how to do it in my case.

How to make a query in server.php file and echo it's data at long poll?

I read about many more article about long poll, but I cannot find any good example to solved in my case.

My jQuery in index.php

        function fetch() {
            $.ajax({
                url: 'http://bdshowbiz.com/server/server.php',
                async: true,
                type: 'get',
                dataType: 'json',
                success: function(ret, textStatus, jqXHR) {
                    $('#response').append('<p>' + ret.aaa + '</p>');
                    fetch();
                },
                error: function(jqXHR, textStatus, errorThrown) {
                    setTimeout(fetch, 5000);
                }
            });
        }
        $(function(){
            fetch();
        });

Current server.php file after implement query(may be it's wrong way)

include("../db.php");
global $dbh;
header('Content-Type: application/json; charset=utf-8');
while (true) {
    //fetch data
    $results = mysqli_query($dbh,"SELECT * FROM comments_lite WHERE qazi_id='1012' ORDER BY date DESC LIMIT 1") or die(mysqli_error($dbh));
    $rows =  mysqli_fetch_assoc($results);
    $data = $rows['description'];

    if (!empty($data)) {
        echo json_encode($data);
        flush();
        exit(0);
    }
    sleep(5);
}