AJAX SET INTERVAL - php

Question

I want to get the latest post_id in the table without refreshing it, but the problem is whenever a user inserts a value to the database, It echoes infinitely the last post_id. I want it to echo only once. But I still want to get the latest post_id from the table.
Here is my main php:
<div id = "this_div">
<?php
include 'connect.php';
session_start();
$query = "SELECT post_id FROM tbl_posts ORDER BY post_id ASC LIMIT 20";
$execute_query = mysqli_query($con,$query);
while($row = mysqli_fetch_assoc($execute_query))
{
$get_this_id = $row['post_id'];
echo $get_this_id."<br>";
}
$_SESSION['get_this_id'] = $get_this_id;
?>
</div>
here is my jQuery ajax:
<script>
var refreshId = setInterval(function(){
compare_session = "<?php echo $_SESSION['get_this_id']; ?>";
$.ajax({
url: 'another_file.php',
data: {},
success: function(data)
{
if(compare_session != data)
{
$('#this_div').text($('#this_div').text()+data);
}
}
});
},400);
</script>
here is the php code of another_file.php
<?php
include 'connect.php';
session_start();
$query = "SELECT post_id FROM tbl_posts ORDER BY post_id DESC LIMIT 1";
$execute_query = mysqli_query($con,$query);
if($row = mysqli_fetch_assoc($execute_query))
{
echo $get_this_id = $row['post_id'];
}
?>

Answer 1

You are not updating the compare_session variable , it holds always the initial value . So update it inside success callback function
compare_session = "<?php echo $_SESSION['get_this_id']; ?>";
var refreshId = setInterval(function () {
$.ajax({
url: 'another_file.php',
data: {},
success: function (data) {
if (compare_session != data) {
$('#this_div').text($('#this_div').text() + data);
}
compare_session = data;
}
});
}, 400);