<?php
session_start();  
  
function getUsersOnline() {  
   $count = 0;  
  
   $handle = opendir(session_save_path());  
   if ($handle == false) return -1;  
  
   while (($file = readdir($handle)) != false) {  
       if (preg_match("/^sess/", $file)) $count++;  
   }  
   closedir($handle);  
  
   return $count;  
}  
?>

$usercount = getUsersOnline();
echo $usercount;