Base10 to base64 url shortening

Question

I'm coding an url shortener function for a project in which I'm learning php, here is the code (btw I suppose that global here is not a good thing to do :P):

$alphabet = array(1 => "a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z",
                "A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z",
                "0","1","2","3","4","5","6","7","8","9","_","-");

function shorten($id){
    global $alphabet;
    $shortenedId = "";
    while($id>0){
        $remainder = $id % 64;
        $id = $id / 64;     
        $shortenedId = $alphabet[$remainder].$shortenedId;
    }
    return $shortenedId;
}

The code is taken from this Wikipedia article and adapted to php. My problem is that when I pass a multiple of 64 to the function I get a wrong (for my purpose) result, for instance 128 returns b which is not correct, it should have been aaa, but that's too long for a 3-digit number.

Also I'm starting to think that there's something wrong in this code, if I pass 1'000'000'000'000 as $id I get nItOq... I feel it's wrong because a url shortening service like bit.ly returns a 6 number id if I use it, and I don't think that this algorithm is better than theirs.

So, two questions:

• do you spot any bug in the above code?
• how to manage 64-multiple ids? Do I have to just ignore them and pass to the next one?

Answer 1

How about this:

function shorten_int($id){
    $hex = base_convert(id, 10, 16);
    $base64 = base64_encode(pack('H*', $hex));
    //$base64 = str_replace("/", "_", $base64); // remove unsafe url chars
    //$base64 = str_replace("+", "-", $base64);
    //$base64 = rtrim($base64, '='); // Remove the padding "=="
    $replacePairs = array('/' => '_',
                          '+' => '-',
                          '=' => '');
    $base64 = strtr($base64, $replacePairs); // optimisation
    return $base64;
}