<?php
error_reporting(-1);
$cities = [
		["name" => "Джакарта", "country" => "Индонезия", "population" => "30539000"],
		["name" => "Токио", "country" => "Япония", "population" => "37843000"],
		["name" => "Шанхай", "country" => "КНР", "population" => "23416000"],
		["name" => "Нью-йорк", "country" => "США", "population" => "20630000"],
		["name" => "Москва", "country" => "Россия", "population" => "16170000"],
		["name" => "Дели", "country" => "Индия", "population" => "24998000"],
		["name" => "Мехико", "country" => "Мексика", "population" => "20063000"],
		["name" => "Стамбул", "country" => "Индия", "population" => "13287000"],
		["name" => "Карачи", "country" => "Пакистан", "population" => "22123000"],
		["name" => "Лондон", "country" => "Великобритания", "population" => "10236000"],
		
	];
	

function filterByPopulation($cities) {	//эту функцию было легко написать
	return array_filter($cities, function($array) {
		return ($array["population"] > 20000000);
	});
}

function printCities($cities) {  //это просто что бы не копипастить завернул.
	array_map(function($city) {
		echo " ", $city["name"] . "(" . $city["population"] . ")";
	}, $cities);
	echo "\n\n";
}


$firstTask = filterByPopulation($cities);
echo "Города с населением больше 20млн:";
printCities($firstTask);


function sortByPopulation($cities) {  //вот тут у меня был затуп
	$condition = function ($a, $b) {  //пришлось воровать пример с мануала и "переделывать".
	    if ($a["population"] == $b["population"]) {
	    	return 0;
    	}
    	return ($a["population"] > $b["population"]) ? -1 : 1;
	};
	usort($cities, $condition);
	return array_slice($cities, 0, 3);
}

$secondTask = sortByPopulation($cities);
echo "Список 3х самых населенных городов по убыванию:";
printCities($secondTask);

$test = array_merge($firstTask, $secondTask);
$result = array_column($test, 'country');
$result = array_unique($result);
echo "Они находятся в странах: " . implode(", ", $result). ".";