Redirect to a different link every 2 hours - cron+txt file

Redirect.php
<?PHP
$sites=array("www.example1.com", "www.example2.com", "www.example2.com", "www.example3.com", "www.example4.com", "www.example5.com", "www.example6.com");
$a=file_get_contents("counter.txt");
header("Location: ".$sites[$a]);
?>

Cron.php
<?PHP
$a=file_get_contents("counter.txt");
$a++;
$f=fopen("counter.txt", "w");
fwrite($f, $a%6); //change the number if you add or remove a site/sites
fclose($f);
?>