如何将此页面计数器更改为唯一的页面计数器

I'm wanting to change this page counter into a unique page counter, I'm not that good a php and need some help.

counter.php file:

<?php 
$datei = fopen("countlog.txt","r");
$count = fgets($datei,1000);
fclose($datei);
$count=$count + 1 ;
echo "$count";
echo " hits";
echo "
";

$datei = fopen("countlog.txt","w");
fwrite($datei, $count);
fclose($datei);

countlog.txt file contains the number of page views and i link counter.php in index.php

Thanks in advance