Grouping array with consecutive keys

I've an array like this -

 Array
(
    [16] => 424
    [17] => 404
    [18] => 416
    [21] => 404
    [22] => 456
    [23] => 879
    [28] => 456
    [29] => 456
    [32] => 123
    [35] => 465
)

The output of this array would be

Array
(
    [0] => Array
           (   ['start'] => 16
               ['stop'] => 19
           )
    [1] => Array
           (   ['start'] => 21
               ['stop'] => 24
 开发者_JS百科          )
    [2] => Array
           (
               ['start'] => 28
               ['stop'] => 30
           )
    [3] => Array
           (
               ['start'] => 32
               ['stop'] => 33
           )
    [4] => Array
           (
               ['start'] => 35
               ['stop'] => 36
           )
)

I don't really need the values. Just grouping the keys.

The 'start' value should be the 'start' value itself. Whereas, the 'stop' value should be a consecutive integer.

And if consecutive integer doesn't exist for a particular key(like for [32] and [35]), 'stop' should be the integer+1 (same as above) .

Thank you all for help.

reset($arr);
$lastKey = key($arr);
$ansIndex = -1;
$ans = array();

foreach ($arr as $k=>$v)
{
    if ($k != $lastKey + 1)
    {
         $ansIndex++;
         $ans[$ansIndex]['start'] = $k; 
    }

    $ans[$ansIndex]['stop']  = $k+1;
    $lastKey = $k;
}

EDIT - changed $k to $k+1 for the stop indexes to reflect change in your question
EDIT - noticed i had a line of code within both if and else. taking it out of conditional since it runs regardless.