I'm trying to refresh data on my web page according to the data stored in the database, so every 2 seconds I call with an ajax request a php file. The called php script is this:

session_start();

.....Connection to the db.......

$prova = pg_query($connect, "SELECT * FROM maxdistance");

$prova2 = "";
while ($row = pg_fetch_row($prova)) {
     $prova2 = $prova2.$row[0].$row[1].$row[2];
}

$_SESSION['prova'] = $prova2;

And this is the code in javascript:

var intervalId = window.setInterval(function(){
  newPositions();
}, 2000);

function newPositions(){
    $(document).ready(function(){
                $.ajax({
                      type: "POST",
                      url: "realTimePosition.php",
                      success: function(msg){
                          provaaa = <?php echo ($_SESSION[prova]); ?>;
                      }
                })
   });

The problem is that, when I refresh the page the code run and in the variable provaaa is stored the value 20 (the actual value in the db) but if I modify the value in the db, the value of the variable is the same, why is this happening?