This is a simple one:
Create a file with the following content within /template/inc_script/frontend_render (and name it rt_users_online):

<?php
/************************************************************************************************************************************
REPTAG CODE: {ONLINE}
 
Developer: Joachim Scholtysik
Homepage: http://www.scholtysik.net
phpwcms Version: 1.3.9
running with r401 too
 
last modified (RCS):  08.03.2010
08.03.2010 (RCS): Fix for some installations - not all will write into the scripts folder
Enhanced output: styling the content
 
18.06.2008: Fix: Changed order of the variables $zeiten and $ips in the array
23.05.2008: Added phpwcms constants check written by O.G.
22.05.2008: Initial release
 
Description:
Shows a status line with users who are online.
 
Installation:
            Copy this file into the frontend_render directory
 
            Create a blank file named "count.txt" and place it also in the frontend_render directory (CHMOD: 777)
 
            Place this line into your template or article:
 
            {ONLINE}
************************************************************************************************************************************/
 
// ----------------------------------------------------------------
// obligate check for phpwcms constants
if (!defined('PHPWCMS_ROOT')) {
   die("You Cannot Access This Script Directly, Have a Nice Day.");
}
// ----------------------------------------------------------------
 
if( ! (strpos($content["all"],'{ONLINE}')===false)) {
   $content["all"] = preg_replace('/\{ONLINE\}/',online(),$content["all"]);
}
 
function online() {
   //$daten = $_SERVER['DOCUMENT_ROOT']."/count.txt";
   $daten = PHPWCMS_ROOT."/count.txt";
   $time  = time();
   $ip    = getenv("REMOTE_ADDR");
   $ablaufzeit = $time - "300";
   $pruefung   = @file($daten);
 
   while (list ($line_num, $line) = @each ($pruefung)) {
      $zeiten = explode("&&",$line);
      if($zeiten[1] <= $ablaufzeit) {
         $fp = fopen( "$daten", "r" );
         $contents = fread($fp, filesize($daten));
         fclose($fp);
         $line=quotemeta($line);
         $string2 = "";
         $replace = ereg_replace($line, $string2, $contents);
         $fh=fopen($daten, "w");
         @flock($fp,2);
         fputs($fh, $replace);
         @flock($fp,3);
         fclose($fh);
      }
   }
   $ippruefung = @file($daten);
   while (list ($line_num, $line) = @each ($ippruefung)) {
      $ips = explode("&&",$line);
      if($ips[0] == $ip) {
         $fp = fopen( "$daten", "r" );
         $contents = fread($fp, filesize($daten));
         fclose($fp);
         $line=quotemeta($line);
         $string2 = "";
         $replace = ereg_replace($line, $string2, $contents);
         $fh=fopen($daten, "w");
         @flock($fp,2);
         fputs($fh, $replace);
         @flock($fp,3);
         fclose($fh);
      }
   }
   $fp = fopen("$daten", "a+");
   flock($fp,2);
   fputs ($fp, "$ip&&$time&&\n");
   flock($fp,3);
   fclose ($fp);
   $anzahldaten = file($daten);
   $anzahl = count($anzahldaten);
   $online="<div style='color:RED;text-align:center'><p><b>$anzahl</b> User online</p></div>";
   return $online;
}
?>

Now create an empty file named “count.txt” and throw it into phpwcms-root (permission 777) and you are done.
Anywhere in your template you can call the script via {ONLINE} or by setting up a CP HTML containig {ONLINE} You can adjust paths and file name anytime - same goes for styling the output.