Two sets of comma separated data - make into one array

I have two sets of data:

$first = "1,4,2,8";
$second = "d,e,r,z";

Is there an easy way of turning this into a keyed array, with the $first set being the keys?

In other words, the resulting array would look like this: array(1=>d, 4=>e, 2=>r, 8=>z)

Thanks!