function convert_from_base62 ($number) {

  $k = 1;
  $result = 0;

  $c = 0;
  $n = strlen ($number);

  while ($c < $n) {

    $d = ord ($number[$c]);

    if (($d >= 48) && ($d <=  57)) $result += $k * ($d - 48);
    if (($d >= 97) && ($d <= 122)) $result += $k * ($d - 97 + 10);
    if (($d >= 65) && ($d <=  90)) $result += $k * ($d - 65 + 36);

    $k *= 62;

    $c ++;

  }

  return $result;

}

function decode ($string) {

  $result = "";
  $c = 0;

  while (1) {

    if ($string[$c] == "") {

      break;

    }

    $sub = substr ($string, $c, 4);
    $sum = convert_from_base62 ($sub);

    $ch1 = ($sum % 225) + 31; if ($ch1 > 31) $result .= chr ($ch1);
    $sum = (int) ($sum / 225);
    $ch2 = ($sum % 225) + 31; if ($ch2 > 31) $result .= chr ($ch2);
    $sum = (int) ($sum / 225);
    $ch3 = ($sum % 225) + 31; if ($ch3 > 31) $result .= chr ($ch3);

    $c += strlen ($sub);

  }

  return $result;

}