<?php
$alpha = array('a', 'b', 'c', 'd', 'e', 'f', 'g');
$count = 0;

if (!isset($_COOKIE['count'])) {
setcookie('count', $count);
}
else {
$count = $_COOKIE['count'] + 1;
setcookie('count', $count);
}

echo $alpha[$count%count($alpha)];
?>