Ajax PHP长轮询

Question

I have this simple long polling:

  function poll(){
         $.ajax({
             ...
             success: function(res){
                $('#count').html(res);
                poll();
             }
        });
     }
 poll();

PHP:

session_start();
$count = $_SESSION['count'];
session_write_close();
while(true){
    session_start();
    $newcount = $_SESSION['count'];
    session_write_close();
    if($newcount != $count){
        $count = $newcount;
        break;
    }
    sleep(1);
}
echo $count;

when ever the count session is updated it break out of the while sleep loop then echoes the new count then polls again. My questions is: is this right? does this code have side effects to the server?