hmm, virker vist ikke for godt......her er en som jeg fandt her på eksperten, den bruger mysql database:
<?php 
/* 
CREATE TABLE userlog ( 
  id2 int(11) DEFAULT \'0\' NOT NULL auto_increment, 
  ID char(255), 
  IP char(255), 
  NAME char(255), 
  COUNT char(255), 
  DATE char(255), 
  PRIMARY KEY (id2) 
); 
*/ 
$database=  \"database\"; 
$table=  \"userlog\"; 
$sqlhost=  \"localhost\"; 
$sqluser=  \"password\"; 
$sqlpass=  \"username\"; 
mysql_connect($sqlhost,$sqluser,$sqlpass) OR DIE(  \"Couldn\'t connect to MySQL server!\"); 
mysql_select_db($database) OR DIE(  \"Couldn\'t select database!\"); 
$IP=$REMOTE_ADDR; 
$HOST= gethostbyaddr($IP); 
$time= explode(  \" \", microtime()); 
$userusec= (double)$time[0]; 
$usersec= (double)$time[1]; 
$deleteuser= mysql_query(  \"delete from userlog where DATE < $usersec -300\"); 
$userlog= mysql_fetch_row(MYSQL_QUERY(  \"SELECT * FROM userlog where IP like \'$IP\'\")); 
if($userlog == false) 
{ 
$ok= @mysql_query(  \"insert INTO userlog (ID,IP,NAME,DATE) VALUES(\'$User_Id\',\'$IP\',\'$HOST\',\'$usersec\')\")or die(  \"Unable to query 
db!\"); 
} 
$resultlogtab  = mysql_query(  \"SELECT Count(*) as total FROM userlog\"); 
$numberlogtab  = mysql_fetch_array($resultlogtab); 
print  \"<p align=\"center\"><font color=\"#FFFFFF\">$numberlogtab[total] User online!</font></p>\"; 
?> 
tjek ellers:
http://www.eksperten.dk/find.phtml?q=online%20phpsom giver dig en del resultater, der er helt sikkert en du kan bruge...