Online PHP editor | output for PB2Bj

<?php $people = [ [1801, 1887], [1943, 1943], [1949, 2001], [1751, 1803], [1881, 1887], [1956, 1983], [1954, 1983], [1801, 1883], [1956, null], [1944, 1983], [1897, 2015], [1965, 1997], [1941, 1982], [1984, null], ]; $populationPerYear = array_reduce( $people, static function($carry, $current) { [$from, $to] = $current; for($i = $from; $i <= $to; $i++) { if (!isset($carry[$i])) { $carry[$i] = 1; } else { $carry[$i]++; } } return $carry; }, [] ); ksort($populationPerYear); $peak = max($populationPerYear); $peakYears = array_filter( $populationPerYear, static function($population) use ($peak) { return $population === $peak; } ); var_dump($peak, $peakYears);