Prueba así:
Código PHP:
<?
include ("config.php");
$t_stamp = time();
$timeout = $t_stamp - $to_secs;
mysql_connect($server, $db_user, $db_pass) or die ("Useronline Database CONNECT Error");
mysql_db_query($db, "INSERT INTO CJ_UsersOnline VALUES ('$t_stamp','$REMOTE_ADDR','$PHP_SELF')") or die("Database INSERT Error");
mysql_db_query($db, "DELETE FROM CJ_UsersOnline WHERE timestamp<$timeout") or die("Database DELETE Error");
$result = mysql_db_query($db, "SELECT DISTINCT ip FROM CJ_UsersOnline WHERE file='$PHP_SELF'") or die("Database SELECT Error");
$user = mysql_num_rows($result);
mysql_close();
echo "<b>$user</b> Usuarios Online";
?>
¿Te importaría poner el código completo? Porque la variable $to_secs no sé dónde está definida
