If you do not know the language, then you are on freelance. If you know, then I will bring an idea: make a table with online users in a database. And when a user visits the site, fill in his id and the time of his visit to it, and each time he visits a page, update the time of his visit. Further, on the online listing page, display all users from the created table, which, for example, has the last visit time less than 2 minutes ago. If more, then delete this entry. If something is not clear, ask.

Well, first, creating a table will look like this:

 CREATE TABLE online ( id int (10) NOT NULL auto_increment, ip varchar (20) NOT NULL default '0', iduser varchar (80) NOT NULL default '0', unix varchar (60) NOT NULL default ', KEY id ( id )) 

The code did not check, there will be errors, write - we will debug it together. Now we need to make all pages

  include 'online.php';
  on_line ();

But if you want to display a table of users online, then so:

  on_line (true); 

Here is the code online.php:

  ------ File online.php 
  / f-I counts users on the line;  returns the number of users in 
  formatted, i.e.  to output the result you only need to register in 
  right place type: echo on_line ();  /
  function on_line ($ echousers = false) {
  $ host = "localhost";  // host where MySql database is located
  $ db_name = "";  // database name;  as a rule coincides with the user name
                 // (variable below), although I am categorically against the same
                 // names, focusing on protection ...
  $ db_user = "";  // user allowed access to the database
  $ db_password = "";  // user password
  $ wine = 300;  // online accuracy (seconds);  time during which
               // user who came to the page, we consider to be
               // Online
  $ table_online = "online";  // table name
 // make global variable IP addresses available
  global $ REMOTE_ADDR;
  // connect to the MySQL server and select the desired database
  mysql_connect ($ host, $ db_user, $ db_password) or die (mysql_error ());
  mysql_select_db ($ db_name) or die (mysql_error ()); 
  // delete all those who have already spent $ wine seconds or who has the current IP
 $ sql_update = "DELETE FROM $ table_online WHERE unix + $ wine <" .time ().
               "OR ip = '$ _SERVER [REMOTE_ADDR]'";
 $ result_update = mysql_query ($ sql_update) or die (mysql_error ()); 
  // insert your post
 $ sql_insert = "INSERT INTO $ table_online (id, ip, iduser, unix)
 $ result_insert = mysql_query ($ sql_insert) or die (mysql_error ()); 
  // consider unique online
 $ sql_sel = "SELECT id FROM $ table_online";
 $ result_sel = mysql_query ($ sql_sel) or die (mysql_error ()); 
  $ online_people = mysql_num_rows ($ result_sel);  // number of On-Line users
 $ online_people = (string) $ online_people;  // cast to string type
                                           // (this is necessary .. see further) 
  $ rain = strlen ($ online_people) - 1;  // number of the last character in the number
                                     // on-line users 
  // formatting output (I did everything for you =)
  if ($ online_people [$ rain] == 2 || $ online_people [$ rain] == 3
 || $ online_people [$ rain] == 4
 || (strlen ($ online_people)! = 1 && $ online_people [strlen ($ online_people) -2]! = 1))
 // $ line - variable that defines the output format
  $ line = "person";  else $ line = "person";
 // return the result
 if ($ echousers == false)
  return "On <strong>". $ online_people. "</ strong> $ line";
 else {
 echo "<table>";
 while ($ row_sel = mysql_fetch_array ($ result_sel)) {
 $ res2 = mysql_query ("SELECT * FROM users WHERE pk_user = '$ row_sel [iduser]'");
 $ row2 = mysql_fetch_array ($ res2);
 echo "<tr> <td>". $ row2 ['flname']. "</ td> </ tr>";
 }
 echo "</ table>";
 }
 }