alpha id generator

alphaidgenerator.php

<?php
/**
 * Translates a number to a short alhanumeric version
 *
 * Translated any number up to 9007199254740992
 * to a shorter version in letters e.g.:
 * 9007199254740989 --> PpQXn7COf
 *
 * specifiying the second argument true, it will
 * translate back e.g.:
 * PpQXn7COf --> 9007199254740989
 *
 * this function is based on any2dec && dec2any by
 * fragmer[at]mail[dot]ru
 * see: http://nl3.php.net/manual/en/function.base-convert.php#52450
 *
 * If you want the alphaID to be at least 3 letter long, use the
 * $pad_up = 3 argument
 *
 * In most cases this is better than totally random ID generators
 * because this can easily avoid duplicate ID's.
 * For example if you correlate the alpha ID to an auto incrementing ID
 * in your database, you're done.
 *
 * The reverse is done because it makes it slightly more cryptic,
 * but it also makes it easier to spread lots of IDs in different
 * directories on your filesystem. Example:
 * $part1 = substr($alpha_id,0,1);
 * $part2 = substr($alpha_id,1,1);
 * $part3 = substr($alpha_id,2,strlen($alpha_id));
 * $destindir = "/".$part1."/".$part2."/".$part3;
 * // by reversing, directories are more evenly spread out. The
 * // first 26 directories already occupy 26 main levels
 *
 * more info on limitation:
 * - http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/165372
 *
 * if you really need this for bigger numbers you probably have to look
 * at things like: http://theserverpages.com/php/manual/en/ref.bc.php
 * or: http://theserverpages.com/php/manual/en/ref.gmp.php
 * but I haven't really dugg into this. If you have more info on those
 * matters feel free to leave a comment.
 *
 * The following code block can be utilized by PEAR's Testing_DocTest
 * <code>
 * // Input //
 * $number_in = 2188847690240;
 * $alpha_in  = "SpQXn7Cb";
 *
 * // Execute //
 * $alpha_out  = alphaID($number_in, false, 8);
 * $number_out = alphaID($alpha_in, true, 8);
 *
 * if ($number_in != $number_out) {
 *    echo "Conversion failure, ".$alpha_in." returns ".$number_out." instead of the ";
 *    echo "desired: ".$number_in."\n";
 * }
 * if ($alpha_in != $alpha_out) {
 *    echo "Conversion failure, ".$number_in." returns ".$alpha_out." instead of the ";
 *    echo "desired: ".$alpha_in."\n";
 * }
 *
 * // Show //
 * echo $number_out." => ".$alpha_out."\n";
 * echo $alpha_in." => ".$number_out."\n";
 * echo alphaID(238328, false)." => ".alphaID(alphaID(238328, false), true)."\n";
 *
 * // expects:
 * // 2188847690240 => SpQXn7Cb
 * // SpQXn7Cb => 2188847690240
 * // aaab => 238328
 *
 * </code>
 *
 * @author   Kevin van Zonneveld <kevin@vanzonneveld.net>
 * @author   Simon Franz
 * @author   Deadfish
 * @copyright 2008 Kevin van Zonneveld (http://kevin.vanzonneveld.net)
 * @license   http://www.opensource.org/licenses/bsd-license.php New BSD Licence
 * @version   SVN: Release: $Id: alphaID.inc.php 344 2009-06-10 17:43:59Z kevin $
 * @link   http://kevin.vanzonneveld.net/
 *
 * @param mixed   $in      String or long input to translate
 * @param boolean $to_num  Reverses translation when true
 * @param mixed   $pad_up  Number or boolean padds the result up to a specified length
 * @param string  $passKey Supplying a password makes it harder to calculate the original ID
 *
 * @return mixed string or long
 */
function alphaID($in, $to_num = false, $pad_up = false, $passKey = null)
{
  $index = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  if ($passKey !== null) {
      // Although this function's purpose is to just make the
      // ID short - and not so much secure,
      // with this patch by Simon Franz (http://blog.snaky.org/)
      // you can optionally supply a password to make it harder
      // to calculate the corresponding numeric ID

      for ($n = 0; $n<strlen($index); $n++) {
          $i[] = substr( $index,$n ,1);
      }

      $passhash = hash('sha256',$passKey);
      $passhash = (strlen($passhash) < strlen($index))
          ? hash('sha512',$passKey)
          : $passhash;

      for ($n=0; $n < strlen($index); $n++) {
          $p[] =  substr($passhash, $n ,1);
      }

      array_multisort($p,  SORT_DESC, $i);
      $index = implode($i);
  }

  $base  = strlen($index);

  if ($to_num) {
      // Digital number  <<--  alphabet letter code
      $in  = strrev($in);
      $out = 0;
      $len = strlen($in) - 1;
      for ($t = 0; $t <= $len; $t++) {
          $bcpow = bcpow($base, $len - $t);
          $out   = $out + strpos($index, substr($in, $t, 1)) * $bcpow;
      }

      if (is_numeric($pad_up)) {
          $pad_up--;
          if ($pad_up > 0) {
              $out -= pow($base, $pad_up);
          }
      }
      $out = sprintf('%F', $out);
      $out = substr($out, 0, strpos($out, '.'));
  } else {
      // Digital number  -->>  alphabet letter code
      if (is_numeric($pad_up)) {
          $pad_up--;
          if ($pad_up > 0) {
              $in += pow($base, $pad_up);
          }
      }

      $out = "";
      for ($t = floor(log($in, $base)); $t >= 0; $t--) {
          $bcp = bcpow($base, $t);
          $a   = floor($in / $bcp) % $base;
          $out = $out . substr($index, $a, 1);
          $in  = $in - ($a * $bcp);
      }
      $out = strrev($out); // reverse
  }

  return $out;
}

Storing numbers inside other numbers in PHP (e.g. '000' --> 101582); what could possibly go wrong?

Comes from https://raw.githubusercontent.com/kvz/deprecated/kvzlib/php/functions/alphaID.inc.php