PHPで数値IDと文字列IDを相互に変換する方法

http://v.youku.com/v_show/id_XNjA5MjE5OTM2.html

<?php
/** 
 * @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&#39;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);
      }
 
      $passhash = hash(&#39;sha256&#39;,$passKey);
      $passhash = (strlen($passhash) < strlen($index))
          ? hash(&#39;sha512&#39;,$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  < 0) {
              $out -= pow($base, $pad_up);
          }
      }
      $out = sprintf(&#39;%F&#39;, $out);
      $out = substr($out, 0, strpos($out, &#39;.&#39;));
  } else {
      // Digital number  -->>  alphabet letter code
      if (is_numeric($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;
}

<?php
alphaID(9007199254740989);

<?php
alphaID(&#39;fE2XnNGpF&#39;, true);