url shortener built on custom base system

gistfile1.php

// Thanks: http://snook.ca/archives/php/url-shortener

$codeset = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
$base = strlen($codeset);
$n = 300;
$converted = "";

while ($n > 0) {
  $converted = substr($codeset, ($n % $base), 1) . $converted;
  $n = floor($n/$base);
}

echo $converted; // 4Q

$c = 0;
for ($i = strlen($converted); $i; $i--) {
  $c += strpos($codeset, substr($converted, (-1 * ( $i - strlen($converted) )),1)) 
        * pow($base,$i-1);
}
echo $c; // 300