Array
(
[0] => Array
(
[date] => 01.09.2016
[sum] => 450
)
[1] => Array
(
[date] => 31.08.2016
[sum] => 156
)
[2] => Array
(
[date] => 02.09.2016
[sum] => 888
)
)
array_map(function($n) {
return [
'date' => $n[0],
'sum' => $n[1]
];
}, array_chunk($arr, 2))