PHP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：

echo alphaID(12354);  //qnd

echo alphaID('qnd',true); //12354

echo alphaID(12354,false,6); //qndaab

echo alphaID('qndaab',true, 6); //12354 

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 <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;

}