<?php
$timeoutseconds = 300;

$timestamp = time();
$timeout = $timestamp - $timeoutseconds;

mysql_connect();
mysql_select_db('users');

$insert = mysql_query(
  "INSERT INTO online VALUES ('$timestamp', '$REMOTE_ADDR', '$PHP_SELF')");

$delete = mysql_query(
  "DELETE FROM online WHERE timestamp < $timeout");

$result = mysql_query(
  "SELECT DISTINCT ip FROM online WHERE file = '$PHP_SELF'");

$users = mysql_num_rows($result);
?>