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：
[U]复制代码[/U] 代码如下:
   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：
[U]复制代码[/U] 代码如下:
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
* @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 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;
}
            
            
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