<?
/*
Database table scheme:

CREATE TABLE usersonline (
timestamp int(15) DEFAULT '0' NOT NULL,
ip varchar(40) NOT NULL,
file varchar(100) NOT NULL,
INDEX (timestamp),
INDEX ip(ip),
INDEX file(file)
);
*/

class UsersOnline {

/* public: connection parameters */
var $host = 'localhost';
var $database = 'websapp';
var $user = '';
var $password = '';

var $timeoutSeconds = 120;
var $numberOfUsers = 0;

function UsersOnline() {
$this->refresh();
}

function getNumber() {
return $this->numberOfUsers;
}

function printNumber() {
if($this->numberOfUsers == 1) {
echo "$this->numberOfUsers User online";
} else {
echo "$this->numberOfUsers Users online";
}
}

function refresh() {
global $REMOTE_ADDR, $PHP_SELF;

$currentTime = time();
$timeout = $currentTime - $this->timeoutSeconds;

mysql_connect($this->host, $this->user, $this->password)
or die('Error conecting to database');

mysql_db_query($this->database,
"INSERT INTO usersonline VALUES ('$currentTime','$REMOTE_ADDR','$PHP_SELF')")
or die('Error writing to database');

mysql_db_query($this->database,
"DELETE FROM usersonline WHERE timestamp < $timeout")
or die('Error deleting from database');

$result = mysql_db_query($this->database,
"SELECT DISTINCT ip FROM usersonline WHERE file='$PHP_SELF'")
or die('Error reading from database');

$this->numberOfUsers = mysql_num_rows($result);

mysql_close();
}

}

?>

위 파일을 online.php로 저장하시고...
원하시는 페이지 부분에...

<?php
include('online.php');

$ol = new UsersOnline();

$ol->printNumber();

?>
이렇게 삽입하시면 됩니다.
위 파일은 php+mysql연동이므로 반드시 php액세스, 및 MySQL사용이 가능해야 합니다.