php base58 decode

function base58_decode($num) {
    $alphabet = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ';
    $len = strlen($num);
    $decoded = 0;
    $multi = 1;
 
    for ($i = $len - 1; $i >= 0; $i--) {
        $decoded += $multi * strpos($alphabet, $num[$i]);
        $multi = $multi * strlen($alphabet);
    }
 
    return $decoded;
}