Does not work perfectly for Online User in WordPress - php

Question

I am making a classified website in wordpress with classipress theme,
http://www.mohibsalahuddin.com/client/ads/test-ad/
for showing author is online or offline on ad page, i wrote the following code, but it does not work if two users registered at a time... If registered user wants to see another registered user's status it always shows offline...
I though may be i missed session start thing do i have to write it? Kindly help me in this
I write the code below
First I create the table in MYSQL
CREATE TABLE wordpress_users_online` (
user_id int(11) default NULL,
full_name varchar(64) NOT NULL default '',
session_id varchar(128) NOT NULL default '',
ip_address varchar(15) NOT NULL default '',
time_entry varchar(14) NOT NULL default '',
time_last_click varchar(14) NOT NULL default '',
last_page_url varchar(255) NOT NULL default ''
) ENGINE=MyISAM;`
THEN I WRITE THE FILE wordpress_users_online.php with following code
<?php
define('TABLE_USERS', 'wordpress_users');
define('TABLE_USER_ONLINE', 'wordpress_users_online');
function update_user_online() {
//global $user_id;
global $current_user;
get_currentuserinfo();
if ($current_user->ID && $current_user->user_login) {
$wo_user_id = $current_user->ID;
$wo_full_name = $user['user_nicename'];
} else {
$wo_user_id = '';
$wo_full_name = 'Guest';
}
$wo_session_id = session_id();
$wo_ip_address = getenv('REMOTE_ADDR');
$wo_last_page_url = getenv('REQUEST_URI');
$current_time = time();
$xx_mins_ago = ($current_time - 300);
// remove entries that have expired
mysql_query("delete from " . TABLE_USER_ONLINE . " where time_last_click < '" . $xx_mins_ago . "'");
$stored_user_query = mysql_query("select count(*) as count from " . TABLE_USER_ONLINE . " where session_id = '" . mysql_escape_string($wo_session_id) . "'");
$stored_user = mysql_fetch_array($stored_user_query);
if ($stored_user['count'] > 0) {
mysql_query("update " . TABLE_USER_ONLINE . " set user_id = '" . (int)$wo_user_id . "', full_name = '" . mysql_escape_string($wo_full_name) . "', ip_address = '" . mysql_escape_string($wo_ip_address) . "', time_last_click = '" . mysql_escape_string($current_time) . "', last_page_url = '" . mysql_escape_string($wo_last_page_url) . "' where session_id = '" . mysql_escape_string($wo_session_id) . "'");
} else {
mysql_query("insert into " . TABLE_USER_ONLINE . " (user_id, full_name, session_id, ip_address, time_entry, time_last_click, last_page_url) values ('" . (int)$wo_user_id . "', '" . mysql_escape_string($wo_full_name) . "', '" . mysql_escape_string($wo_session_id) . "', '" . mysql_escape_string($wo_ip_address) . "', '" . mysql_escape_string($current_time) . "', '" . mysql_escape_string($current_time) . "', '" . mysql_escape_string($wo_last_page_url) . "')");
}
}
function is_online($user_id) {
$stored_user_query = mysql_query("select user_id from ".TABLE_USER_ONLINE." where user_id = '" . mysql_escape_string($user_id) . "'");
$stored_user = mysql_fetch_array($stored_user_query);
if(isset($stored_user['user_id'])){
$online = '<img src="http://www.mohibsalahuddin.com/client/wp-content/themes/classipress-313/images/online.png" border="0" /> Online';
echo $online;
}
else {
$online = '<img src="http://www.mohibsalahuddin.com/client/wp-content/themes/classipress-313/images/offline.png" border="0" /> Offline';
echo $online;
}
return $online;
}
?>

Answer 1

There's actually a wordpress plugin for this already...
--Edit--
http://wordpress.org/extend/plugins/wp-useronline/
Sorry the following was the original suggestions but they were the wrong links:
http://wordpress.org/extend/plugins/who-is-online/faq/
http://wordpress.org/extend/plugins/wassup/
Remember, don't re-invent the wheel unless you're going to make it hover too.