How many visitors are currently online:) How can I do it in PHP?

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How many visitors are currently online How can I do it in PHP? I want to know how much visitors are currently online on my web site. It is possible to do one script in PHP that show it in my website?

 

[dodita]

Here is my code, remember to add the db by yourself.
Hope it'd help you out

function online() {
$ip = getenv("REMOTE_ADDR"
$past = time()-900;
mysql_query(&quot;DELETE FROM session WHERE time < $past&quot;
$result = mysql_query(&quot;SELECT time FROM session WHERE host_addr='$ip'&quot;
$ctime = time();
if ($row = mysql_fetch_array($result)) {
mysql_query(&quot;UPDATE session SET username='time='$ctime', host_addr='$ip' WHERE host_addr='$ip'&quot;
} else {
mysql_query(&quot;INSERT INTO session (time, host_addr) VALUES ('$ctime', '$ip')&quot;
}

$result = mysql_query(&quot;SELECT host_addr FROM session&quot;
$who_online_num = mysql_num_rows($result);
echo = &quot;<font size='1' face='verdana' color='#000000'>( $who_online Currently Online )</font>&quot;;
}

 

[Guest_imported]

Special Thanks)