Online PHP editor | refs for Gi1Zi

<?php $births = [1997, 1997, 1997, 1998, 1999]; $deaths = [1998, 1999]; $years = []; foreach ($births as $year) { if (!isset($years[$year])) $years[$year] = 0; $years[$year]++; } foreach ($deaths as $year) { if (!isset($years[$year])) $years[$year] = 0; $years[$year]--; } ksort($years); $acc = 0; $maxYear = null; $maxYearValue = null; foreach ($years as $year => $value) { if ($acc !== 0) { $years[$year] = $value + $acc; } $acc = $acc + $value; if ($acc > $maxYearValue) { $maxYear = $year; $maxYearValue = $acc; } } var_dump($years, $maxYear, $maxYearValue);