Unique ID URL names (short URL)

function alphaID($in, $to_num = false, $pad_up = false, $passKey = null)
{
    $index = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    if ($passKey !== null)
    {

        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;
}


Example:
alphaID(9007199254740989);   //-> PpQXn7COf
alphaID('PpQXn7COf', true);  //-> 9007199254740989