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<?php $array = array( "date_2019_12" => 0, "date_2019_11" => 0, "date_2019_10" => 0, "date_2019_09" => 0, "date_2019_08" => 0, "date_2019_07" => 0, "date_2019_06" => 0, "date_2019_05" => 0, "date_2019_04" => 0, "date_2019_03" => 0, "date_2019_02" => 0, "date_2019_01" => 10, "date_2018_12" => 1, "date_2018_11" => 2, "date_2018_10" => 3, "date_2018_09" => 4, "date_2018_08" => 5, "date_2018_07" => 6, ); krsort($array); $year = "2019"; $keys = array_keys($array); foreach(array_values($array) as $k => $v){ if(strpos($keys[$k], $year) !== false) $res[$keys[$k]] = array_sum(array_slice($array, $k, 6)); } var_dump($res);
Output for 7.1.25 - 7.1.28, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
array(12) { ["date_2019_12"]=> int(0) ["date_2019_11"]=> int(0) ["date_2019_10"]=> int(0) ["date_2019_09"]=> int(0) ["date_2019_08"]=> int(0) ["date_2019_07"]=> int(0) ["date_2019_06"]=> int(10) ["date_2019_05"]=> int(11) ["date_2019_04"]=> int(13) ["date_2019_03"]=> int(16) ["date_2019_02"]=> int(20) ["date_2019_01"]=> int(25) }

preferences:
159.12 ms | 404 KiB | 162 Q