How to make a value in php update every secound

I want the "Visitors online: 001" on my stats.php site to update the visitors 5 secounds. it works if i reload the page and have another device with a different ip with the page open. I would really appreciate some help with this. it could be a small code i could include that i can change to update the counter or anything will help me.

<?php
/*************************************************************************
php easy :: online visitors counter scripts set - PHP Include Version
==========================================================================
Author: php easy code, www.phpeasycode.com
Web Site: http://www.phpeasycode.com
Contact: webmaster @ phpeasycode.com
*************************************************************************/
$servername = "localhost";
$username = "non-root";
$password = "123";
$dbname = "db";
// Connection to database
$conn = mysqli_connect($servername, $username, $password, $dbname);
// Check connection
  if (mysqli_connect_errno())
    {
    echo 'NOT_OK';
    //echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }

$dbfile = "visitors.db"; // path to data file
$expire = 300; // average time in seconds to consider someone online before removing from the list

if(!file_exists($dbfile)) {
die("Error: Data file " . $dbfile . " NOT FOUND!");
}

if(!is_writable($dbfile)) {
die("Error: Data file " . $dbfile . " is NOT writable! Please CHMOD it to 666!");
}

function CountVisitors() {
global $dbfile, $expire;
$cur_ip = getIP();
$cur_time = time();
$dbary_new = array();

$dbary = unserialize(file_get_contents($dbfile));
if(is_array($dbary)) {
while(list($user_ip, $user_time) = each($dbary)) {
if(($user_ip != $cur_ip) && (($user_time + $expire) > $cur_time)) {
$dbary_new[$user_ip] = $user_time;
}
}
}
$dbary_new[$cur_ip] = $cur_time; // add record for current user

$fp = fopen($dbfile, "w");
fputs($fp, serialize($dbary_new));
fclose($fp);

$out = sprintf("%03d", count($dbary_new)); // format the result to display 3 digits with leading 0's
return $out;
}

function getIP() {
if(isset($_SERVER['HTTP_X_FORWARDED_FOR'])) $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
elseif(isset($_SERVER['REMOTE_ADDR'])) $ip = $_SERVER['REMOTE_ADDR'];
else $ip = "0";
return $ip;
}

$visitors_online = CountVisitors();
?>
<p>Visitors online: <b><?=$visitors_online;?></b></p>
?>