Конвертация числа из десятичной системы в любую

<?php
class BaseConvert {
    private static $base = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

    public static function encode($num, $b = 62) {
        $r = $num % $b;
        $res = self::$base[$r];
        $q = floor($num / $b);
        while ($q) {
            $r = $q % $b;
            $q = floor($q / $b);
            $res = self::$base[$r] . $res;
        }
        return $res;
    }

    public static function decode($num, $b = 62) {
        $limit = strlen($num);
        $res = strpos(self::$base, $num[0]);
        for ($i = 1; $i < $limit; $i++) {
            $res = $b * $res + strpos(self::$base, $num[$i]);
        }
        return $res;
    }
}
var_dump(BaseConvert::encode(124, 36), BaseConvert::decode('3g', 36));

<?php
var_dump(
base_convert(124, 10, 36),
base_convert('3g', 36, 10)
);