Online PHP editor | refs for o6Yni

<?php $array = [ '2015-09-23' => [ 'user' => [13, 12], 'count' => [ 'count' => 2, 'total' => 2, ], ], '2015-09-24' => [ 'user' => [14], 'count' => [ 'count' => 1, 'total' => 1, ], ], '2015-09-25' => [ 'user' => [15], 'count' => [ 'count' => 1, 'total' => 1, ], ], '2015-09-26' => [ 'user' => [16], 'count' => [ 'count' => 1, 'total' => 1, ], ], '2015-09-27' => [ 'user' => [17, 18], 'count' => [ 'count' => 2, 'total' => 2, ], ], '2015-09-28' => [ 'user' => [19, 20, 21, 22, 23, 24, 25, 26, 27, 28], 'count' => [ 'count' => 10, 'total' => 10, ], ], ]; var_export( array_map( function ($set) { static $prev = 0; $set['count']['total'] += $prev; $prev = $set['count']['total']; return $set; }, $array ) );