Uniqueness in ID

Question

I am using the first half code below to generate a unique ID. Then i am using the entry generated in the first half as a variable for the second piece of code that generates another unique ID that is shorter, will the code i am using guarantee 100% uniqueness. I am new so please bear with me:

PHP:

// 1st Piece of code

// Generate Guid 
function NewGuid() { 
    $s = strtoupper(uniqid(rand(),true)); 
    $guidText = 
        substr($s,0,8) . '-' . 
        substr($s,8,4) . '-' . 
        substr($s,12,4). '-' . 
        substr($s,16,4). '-' . 
        substr($s,20); 
    return $guidText;
}
// End Generate Guid 

$Guid = NewGuid();

//2nd Piece of code

$alphabet = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ';    

function base_encode($num, $alphabet) {
    $base_count = strlen($alphabet);
    $encoded = '';

    while ($num >= $base_count) {

        $div = $num/$base_count;
        $mod = ($num-($base_count*intval($div)));
        $encoded = $alphabet[$mod] . $encoded;
        $num = intval($div);
    }

    if ($num) $encoded = $alphabet[$num] . $encoded;
        return $encoded;
}


function base_decode($num, $alphabet) {
    $decoded = 0;
    $multi = 1;

    while (strlen($num) > 0) {
        $digit = $num[strlen($num)-1];
        $decoded += $multi * strpos($alphabet, $digit);
        $multi = $multi * strlen($alphabet);
        $num = substr($num, 0, -1);
    }

    return $decoded;
}

ob_start();
echo base_encode($Guid, $alphabet); //should output: bUKpk
$ider = ob_get_contents();
ob_end_clean();

Answer 1

Will the code i am using guarantee 100% uniqueness?

Of course it's not possible to 100% guarantee uniqueness with a random GUID. It's very likely that each GUID will be unique, but there is only a finite (but very large) number of GUIDs. If you generate enough of them you will eventually run out of unused GUIDs and be forced to generate a duplicate. And the birthday paradox means that randomly choosing GUIDs will on average give you a duplicate sooner than you might think.

But it's probably not worth worrying about in practice as even with the birthday paradox, it is still extremely unlikely to happen.