Reduce length of a number

NumberShorter.php

<?php

namespace Aloha;

class NumberShorten
{
//    readable character set excluded (0,O,1,l)
//    const CODESET = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const CODESET = "23456789abcdefghijkmnopqrstuvwxyzABCDEFGHIJKLMNPQRSTUVWXYZ";

    public function shorten($number){
        $base = strlen(self::CODESET);
        $converted = "";

        while ($number > 0) {
            $converted = substr(self::CODESET, ($number % $base), 1) . $converted;
            $number = floor($number/$base);
        }

        return $converted;
    }

    function unshorten($shortened){
        $base = strlen(self::CODESET);
        $c = 0;
        for ($i = strlen($shortened); $i; $i--) {
            $c += strpos(self::CODESET, substr($shortened, (-1 * ( $i - strlen($shortened) )),1)) * pow($base,$i-1);
        }
        return $c;
    }
}