Mine

<?php

$vars['start']   = $_GET['start'];
$vars['end']     = $_GET['end'];
$vars['charset'] = $_GET['charset'];

if($vars['charset'] == '26') {
  $chars = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
} else {
  echo "Invalid charset";
  exit;
}

foreach(range(1,8) as $key) {
  foreach(range(1,$key) as $tracker) {
    $intervals[$key] += pow($vars['charset'], $tracker);
  }
}

echo "<pre>";
print_r($vars);
print_r($intervals);
echo "</pre>";

function find_length() {

}

while ($vars['start'] < $vars['end']) {
  $length = find_length($start,$finish);
}

?>