PHP实现把数字ID转字母ID

ID是网站中经常出现的，它一般是数字，但是我们发现现在的网站很多ID都是字母了,比如YouTube的视频播放页它的URL类似/watch?v=yzNjIBEdyww。 下面是一个生成字母ID的方法。
使用示例：

   alphaID(12354）；  //会将数字转换为字母。
   alphaID('PpQXn7COf',true）；//会将字母ID转换为对应的数字。
   alphaID(12354,false,6）；//指定生成字母ID的长度为6.

源码：

<?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.
 *
 * @author  Kevin van Zonneveld <[e-mail]>
 * @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;
}