anyone know a simple code to show when a user is online?

<?php

$file_name = "whois_online.dat.php";
$c_time = time();
$timeout = 300;
$time = $c_time - $timeout;
$ip = getenv("REMOTE_ADDR");

if(!file_exists($file_name)){
$fp = fopen($file_name, "w");
fwrite($fp, "<?php die('Restricted File');?> \n");
fclose($fp);
}


$fp = fopen($file_name, "a");
$write = $ip."||".$c_time."\n";
fwrite($fp, $write);
fclose($fp);


$file_array = file($file_name);
$online_array = array();
for($x = 1; $x < count($file_array); $x++){
list($ip, $ip_time) = explode("||", $file_array[$x]);
if($ip_time >= $time){
array_push($online_array, $ip);
}
}

$online = array_unique($online_array);
$online = count($online);
if($online == "1"){
print "User online: $online"."<br>";
}else{
print "Users online: $online"."<br>";
}
?>