convert id between number systems

NumberSystem.php

class NumberSystem
{
    const DEFAULT_ALPHABET = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ';

    public static function baseEncode($num, $alphabet = self::DEFAULT_ALPHABET) {
        $baseCount = strlen($alphabet);
        $encoded = '';
        while ($num >= $baseCount) {
            $div = $num/$baseCount;
            $mod = ($num-($baseCount* (int) $div));
            $encoded = $alphabet[$mod] . $encoded;
            $num = (int) $div;
        }

        if ($num) {
            $encoded = $alphabet[$num] . $encoded;
        }

        return $encoded;
    }

    public static function baseDecode($num, $alphabet = self::DEFAULT_ALPHABET) {
        $decoded = 0;
        $multi = 1;
        while (strlen($num) > 0) {
            $digit = $num[strlen($num)-1];
            $decoded += $multi * strpos($alphabet, $digit);
            $multi = $multi * strlen($alphabet);
            $num = substr($num, 0, -1);
        }

        return $decoded;
    }
}