user_online.php

<?php
    if(isset($_SESSION['user'])){
        $sqlm=$dbh->prepare("SELECT name FROM chatters WHERE name=?");
        $sqlm->execute(array($_SESSION['user']));

        if($sqlm->rowCount()!=0){
            $sql=$dbh->prepare("UPDATE chatters SET seen=NOW() WHERE name=?");
            $sql->execute(array($_SESSION['user']));
        }else{
            $sql=$dbh->prepare("INSERT INTO chatters (name,seen) VALUES (?,NOW())");
            $sql->execute(array($_SESSION['user']));
        }
    }

    //pastikan wakti pada database dan php server sama
    $sql=$dbh->prepare("SELECT * FROM chatters");
    $sql->execute();

    while($r=$sql->fetch()){
        $curtime=strtotime(date("Y-m-d H:i:s",strtotime('-25 seconds', time())));

        if(strtotime($r['seen']) < $curtime){
            $kql=$dbh->prepare("DELETE FROM chatters WHERE name=?");
            $kql->execute(array($r['name']));
        }
    }
?>