How do I know the online status of each member? Please change the code by "I".

I am writing hard

$data = $pdo->query(
          "SELECT * FROM `user` "
        );
        $data2 = $pdo->query(
          "SELECT * FROM `user` "
        );

where to display

<? while($row = mysqli_fetch_array($data)){ ?>
<div class="user-<?=$row["id"];?>"></div>

jquery and backend
this is at the bottom of the network, because my jquery account is below
or he will say that he can"t find $
, so

<script src="jquery.js"></script>

<? while($row = mysqli_fetch_array($data2)){ ?>

  <script>
  online_status_<?=$row["id"];?>();
  function online_status_<?=$row["id"];?>(){
    $(".user-<?=$row["id"];?>").load("/notify?type=xxx&id=<?=$row["id"];?>", function(data){
      $(".user-<?=$row["id"];?>").html(data);
    });
    setTimeout(online_status_<?=$row["id"];?>, 10000);
  };
  </script>
<?}?>

backend

$data = $pdo->query(
      "SELECT * FROM `user`
      WHERE
        `id` = "{$_GET["id"]}" "
    );
    if(mysqli_num_rows($data)!=0){

      $row = mysqli_fetch_array($data);
      if(time()-$row["online_status"] > 20){
        echo "";
      }else{
        echo "";
      }
    }

The

user number table has an online_status bit
I update this bit every ten seconds, stamped with a timestamp.
is here to determine whether the upper and lower lines if (time ()-$row ["online_status"] > 20) {
this way is strangely feasible, but definitely not feasible!
but I want to find the Great God. I"ll see how to change the code!