Código PHP:
$username=($_SESSION['s_username']);
$lastlogin=date("d/m/Y h:i A");
$pegar = "SELECT * FROM online ORDER BY username";
$cad = mysql_db_query($dbname,$pegar) or die (mysql_error());
while($row = mysql_fetch_array($cad)){
if ($row[usuario] == $username){
}else{
$sql = "INSERT INTO online (username, lastlogin, ip) VALUES ('$username','$lastlogin', '$ip_real')";
$result = mysql_query($sql);
}
}
Te faltaba un }