Code php for make user online on web/blog

Make file with name:
1. user_online.php

Step by step:
1. Create table "user_online" on mysql on database "test".
2. Create file user_online.php.

Create table "user_online"
CREATE TABLE `user_online` (
`
sessionchar(100NOT NULL default '',
`
timeint(11NOT NULL default '0' TYPE=MyISAM;  

Create file user_online.php
<?
session_start();
$session=session_id();
$time=time();
$time_check=$time-600//SET TIME 10 Minute

$host="localhost"// Host name
$username=""// Mysql username
$password=""// Mysql password
$db_name="test"// Database name
$tbl_name="user_online"// Table name

// Connect to server and select databse
mysql_connect("$host""$username""$password")or die("cannot connect to server");
mysql_select_db("$db_name")or die("cannot select DB");

$sql="SELECT * FROM $tbl_name WHERE session='$session'";
$result=mysql_query($sql);

$count=mysql_num_rows($result);

if($count=="0"){
$sql1="INSERT INTO $tbl_name(session, time)VALUES('$session', '$time')";
$result1=mysql_query($sql1);
}
else {
"$sql2=UPDATE $tbl_name SET time='$time' WHERE session = '$session'";
$result2=mysql_query($sql2);
}

$sql3="SELECT * FROM $tbl_name";
$result3=mysql_query($sql3);

$count_user_online=mysql_num_rows($result3);

echo "User online : $count_user_online ";

// if over 10 minute, delete session
$sql4="DELETE FROM $tbl_name WHERE time<$time_check";
$result4=mysql_query($sql4);

mysql_close();

// Open multiple browser page for result
?>
Just that..