function alphaId($in, $to_num = false, $pad_up = false, $pass_key = null)
    {
        $out   =   '';
        $index = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $base  = strlen($index);

        if ($pass_key !== 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);
            }

            $pass_hash = hash('sha256',$pass_key);
            $pass_hash = (strlen($pass_hash) < strlen($index) ? hash('sha512', $pass_key) : $pass_hash);

            for ($n = 0; $n < strlen($index); $n++) {
                $p[] =  substr($pass_hash, $n, 1);
            }

            array_multisort($p, SORT_DESC, $i);
            $index = implode($i);
        }

        if ($to_num) {
            // Digital number  <<--  alphabet letter code
            $len = strlen($in) - 1;

            for ($t = $len; $t >= 0; $t--) {
                $bcp = bcpow($base, $len - $t);
                $out = $out + strpos($index, substr($in, $t, 1)) * $bcp;
            }

            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);
                }
            }

            for ($t = ($in != 0 ? floor(log($in, $base)) : 0); $t >= 0; $t--) {
                $bcp = bcpow($base, $t);
                $a   = floor($in / $bcp) % $base;
                $out = $out . substr($index, $a, 1);
                $in  = $in - ($a * $bcp);
            }
        }

        return $out;
    }
function alphaId($in, $to_num = false, $pad_up = false, $pass_key = null)
    {
        $out   =   '';
        $index = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $base  = strlen($index);

        if ($pass_key !== 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);
            }

            $pass_hash = hash('sha256',$pass_key);
            $pass_hash = (strlen($pass_hash) < strlen($index) ? hash('sha512', $pass_key) : $pass_hash);

            for ($n = 0; $n < strlen($index); $n++) {
                $p[] =  substr($pass_hash, $n, 1);
            }

            array_multisort($p, SORT_DESC, $i);
            $index = implode($i);
        }

        if ($to_num) {
            // Digital number  <<--  alphabet letter code
            $len = strlen($in) - 1;

            for ($t = $len; $t >= 0; $t--) {
                $bcp = bcpow($base, $len - $t);
                $out = $out + strpos($index, substr($in, $t, 1)) * $bcp;
            }

            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);
                }
            }

            for ($t = ($in != 0 ? floor(log($in, $base)) : 0); $t >= 0; $t--) {
                $bcp = bcpow($base, $t);
                $a   = floor($in / $bcp) % $base;
                $out = $out . substr($index, $a, 1);
                $in  = $in - ($a * $bcp);
            }
        }

        return $out;
    }
function alphaId ( $ in , $ to_num = false , $ pad_up = false , $ pass_key = null ) { $ out = '' ; $ index = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ' ; $ base = strlen ( $ index ) ; if ( $ pass_key !== 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 ) ; } $ pass_hash = hash ( 'sha256' , $ pass_key ) ; $ pass_hash = ( strlen ( $ pass_hash ) < strlen ( $ index ) ? hash ( 'sha512' , $ pass_key ) : $ pass_hash ) ; for ( $ n = 0 ; $ n < strlen ( $ index ) ; $ n ++ ) { $ p [ ] = substr ( $ pass_hash , $ n , 1 ) ; } array_multisort ( $ p , SORT_DESC , $ i ) ; $ index = implode ( $ i ) ; } if ( $ to_num ) { // Digital number  <<--  alphabet letter code $ len = strlen ( $ in ) - 1 ; for ( $ t = $ len ; $ t >= 0 ; $ t -- ) { $ bcp = bcpow ( $ base , $ len - $ t ) ; $ out = $ out + strpos ( $ index , substr ( $ in , $ t , 1 ) ) * $ bcp ; } 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 ) ; } } for ( $ t = ( $ in != 0 ? floor ( log ( $ in , $ base ) ) : 0 ) ; $ t >= 0 ; $ t -- ) { $ bcp = bcpow ( $ base , $ t ) ; $ a = floor ( $ in / $ bcp ) % $ base ; $ out = $ out . substr ( $ index , $ a , 1 ) ; $ in = $ in - ( $ a * $ bcp ) ; } } return $ out ; }
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.

The following code block can be utilized by PEAR's Testing_DocTest
<code>
// Input //
$number_in = 2188847690240;
$alpha_in  = "SpQXn7Cb";

// Execute //
$alpha_out  = alphaID($number_in, false, 8);
$number_out = alphaID($alpha_in, true, 8);

if ($number_in != $number_out) {
echo "Conversion failure, ".$alpha_in." returns ".$number_out." instead of the ";
echo "desired: ".$number_in."\n";
}
if ($alpha_in != $alpha_out) {
echo "Conversion failure, ".$number_in." returns ".$alpha_out." instead of the ";
echo "desired: ".$alpha_in."\n";
}

// Show //
echo $number_out." => ".$alpha_out."\n";
echo $alpha_in." => ".$number_out."\n";
echo alphaID(238328, false)." => ".alphaID(alphaID(238328, false), true)."\n";

// expects:
// 2188847690240 => SpQXn7Cb
// SpQXn7Cb => 2188847690240
// aaab => 238328

</code>

@author  Kevin van Zonneveld &lt;kevin@vanzonneveld.net>
@author  Simon Franz
@author  Deadfish
@author  SK83RJOSH
@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  $pass_key Supplying a password makes it harder to calculate the original ID

@return mixed string or long
Translates a number to a short alhanumeric version