remove last comma from string

The advantage of using rtrim() is that if you have empty positions at the end of the array and wind up with something like this 3,4,5,,, it will declop all of the commas at the end.  Thus it is better than something like substr($str,0,-1).

But I have another suggestion for you.  Why use an iterator at all?  Why not do something like this...

$arr = array(3,4,5,6,2);
$str = implode(',', $arr);

HTH, ~Ray