/**
 * 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 
 * @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   $pad_up Number or boolean padds the result up to a specified length
 * 
 * @return mixed string or long
 */
function alphaID($in, $to_num = false, $pad_up = false)
{
    $index = \"abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ\";
    $base  = strlen($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, $pad_up);
            }
        }
    } else { 
        // Digital number  -->>  alphabet letter code
        if (is_numeric($pad_up)) {
            $pad_up--;
            if ($pad_up > 0) {
                $in += pow($base, $pad_up);
            }
        }
 
        $out = \"\";
        for ($t = floor(log10($in) / log10($base)); $t >= 0; $t--) {
            $a   = floor($in / bcpow($base, $t));
            $out = $out . substr($index, $a, 1);
            $in  = $in - ($a * bcpow($base, $t));
        }
        $out = strrev($out); // reverse
    }
 
    return $out;
}
echo alphaID(27101984); //M2IPb
echo alphaID(\'M2IPb\', TRUE);