##########################################################################################################
#   "Who is Online Hack" coded by LaughingLizard with help from http://www.webkreator.com/php/concepts/  #
#    Compatible with all versions of WordPress	http://www.mindfulmusings.net/weblog/		         #
##########################################################################################################

1) Open up mysql command line tool, and type in these lines:
(this is considering wpdatabase is the name of your wordpress database)

use wpdatabase;
create table usersOnline
(
id int auto_increment not null,
userIP varchar(20) not null,
dateAdded timestamp,
primary key(id),
unique id(id)
);

Or you could use phpMyAdmin or any other MySql admin tool to create a new table called usersOnline
inside your WordPress database with the above columns.

2) If you are using WordPress 1.0 or above AND are using my-hacks.php, copy the function from below to your my-hacks.php
and place it within the <?php and the ?> tags.
If you are using 0.72 or below, copy these to the end of your b2functions.php which is inside your b2include folder;
again make sure that they are within the <?php and the ?> tags.

function whoIsOnline() {
	$maxOnlineTime = 5;  //Change the value for maximum time a user is considered "online"
	global $HTTP_SERVER_VARS;
	$userIP = $HTTP_SERVER_VARS["REMOTE_ADDR"];
	$timeMax = time() - (60 * $maxOnlineTime);
	$result = mysql_query("select id from usersOnline where userIP = '$userIP'");
	while ($row = mysql_fetch_array($result, MYSQL_NUM)) {
		@mysql_query("delete from usersOnline where id = '$row[0]'");
	}
	@mysql_query("insert into usersOnline(userIP) values('$userIP')");
	$timeMax = time() - (60 * $maxOnlineTime);
	$result = mysql_query("select count(*) from usersOnline where unix_timestamp(dateAdded) >= '$timeMax'");
	$usersOnline = mysql_result($result, 0, 0);
	echo "There " . ($usersOnline != 1 ? "are" : "is") . " $usersOnline user" . ($usersOnline != 1 ? "s" : "") . " online";
	}

3) Now insert this code anywhere inside your index.php (outside the wp-loop) to get number of users online

<?php whoIsOnline(); ?>

All done!

PS: If you get thousands of hits a day, you will need to clean out that table (usersOnline) now and then to reduce load on your MySql server.