add_action('publish_listings', 'vba_update', 10, 1);
add_action('draft_listings', 'vba_update', 10, 1);
function vba_update($post_id){


$args = array(
    'include'   => tb_get_followers($user_id)
            );
$user_query = new WP_User_Query($args);

if(!empty($user_query->results)) :

    $post_title = get_the_title( $post_id );
    $post_url = get_permalink( $post_id );
    $subject = 'A post has been updated';
    $message = "A post has been updated on your website:nn";
    $message.= "<a href='". $post_url. "'>" .$post_title. "</a>nn";


   /* foreach($user_query->results as $user) :

        wp_mail($user->data->user_email, $subject, $message);

    endforeach; */

endif;

}


add_action('wp_ajax_update_notification_db' , 'update_notification_db');
add_action('wp_ajax_nopriv_update_notification_db','update_notification_db');
function update_notification_db(){

    //This script will accept following three commands as GET parameter and will act on as follows:
//INSERT_NOTIFICATION - Insert the notification in the [notifications] table
//FETCH_NOTIFICATIONS - Get all notifications for a user from [notifications] table
//DELETE_NOTIFICATIONS - Deletes all notifications for a user from [notifications] table
//SET_IS_VIEWED_FLAG - Call this command from front-end when user has viewed the notification

$command = "";

if (isset($_GET['command'])) {

    $command = $_GET['command'];

}

$user_id = "";

if (isset($_GET['user_id'])) {

    $user_id = $_GET['user_id'];

}

if (strcmp($user_id,"") == 0) {

    echo "<br>Error: user_id must be specified as GET parameter.";

    exit;
}

if (strcmp($command,"") == 0) {

    echo "<br>Sorry, no command specified. Exiting...";

    exit;

}

//Open the MySQL database connection here and select the database in which [notifications] table has been created
global $wpdb;
$servername = "localhost";
$username = "";
$password = "";
$dbname = "root";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {

    die("Connection failed: " . $conn->connect_error);

}

if (strcmp($command,"INSERT_NOTIFICATION") == 0) {

$notification_text = "";

if (isset($_GET['notification_text'])) {

    $notification_text = $_GET['notification_text'];

}

$is_viewed = "";

if (isset($_GET['is_viewed'])) {

    $is_viewed = $_GET['is_viewed'];

}

if (strcmp($notification_text,"") == 0) {

    echo "<br>Error: notification_text must be specified as GET parameter.";

    exit;

}

if (strcmp($is_viewed,"") == 0) {

    echo "<br>Error: is_viewed must be specified as GET parameter.";

    exit;

}

//Insert notification into the table for given user
$sql = "select * from notifications";

$result = $conn->query($sql);

$id = mysql_num_rows($result);

$id = $id + 1;

$creation_date_time = date("Y:m:d H:i:s");

$view_date_time = date("Y:m:d H:i:s");

$is_viewed = 'NO';

$sql = "INSERT INTO `notifications`(`id`, `creation_date_time`, `view_date_time`, `user_id`, `notification_text`, `is_viewed`) VALUES ('".$id."','".$creation_date_time."','".$view_date_time."','".$user_id."','".$notification_text."','".$is_viewed."')";

$result = $conn->query($sql);

$conn->close();

exit;

}

if (strcmp($command,"FETCH_NOTIFICATIONS") == 0) {
//Select all notifications for a given user_id and return in JSON format
$sql = "select * from notifications where user_id = '".$user_id."' and is_viewed = NO";

$res = mysqli_query($con,$sql);

$result = array();

while($row = mysqli_fetch_array($res)) {

array_push($result,array('id'=>$row[0],'creation_date_time'=>$row[1],'view_date_time'=>$row[2],'user_id'=>$row[3],'notification_text'=>$row[4],'is_viewed'=>$row[5]));

}

$conn->close();

echo json_encode(array("result"=>$result_array));

//Here you can format all notifications into well-organized HTML code just like Facebook in order to present in nice way

//For example purpose , it is being returned in JSON format

exit;

}

if (strcmp($command,"DELETE_NOTIFICATIONS") == 0) {
//Delete all notifications for a given user_id
$sql = "delete from notifications where use_id = '".$user_id."'";

$res = mysqli_query($con,$sql);

$conn->close();

exit;

}

if (strcmp($command,"SET_IS_VIEWED_FLAG") == 0) {

//Update here is_viewed flag to YES for notification and user_id
exit;

}

}

<script  type="text/javascript" charset="utf-8">
function show_notifications() {

var command = '&command=FETCH_NOTIFICATIONS&user_id=1';

$(document).ready(function() {
    $.ajax({
        url: ajaxurl,
        cache: false,
        data: {action: 'update_notification_db' + command},
        success: function(data) {
        console.log(data);
        //$("#notification_results").append(html);
        //Change the content of the DIV in User's profile page where notifications will be displayed
        }
    });

});


}

function startTimer() {
//call show_notifications
show_notifications();
//then start interval
setInterval(show_notifications, 30000); //Refresh after 30 seconds
}
</script>

<script>
function create_notifications() {

var command = '&command=INSERT_NOTIFICATION&user_id=1&creation_date_time=201610101200&view_date_time=2016101012000&notification_text=Hello&is_viewed=NO';

$(document).ready(function() {
    $.ajax({
        url: ajaxurl,
        cache: false,
        data: {action: 'update_notification_db' + command},
        success: function(data){
            console.log(data);
        }
    });
});
}


</script>