array_map / array_filter

<?php
$units = [12, 8, 5];
echo implode(", ", array_map(fn($n) => $n * 40, $units)), PHP_EOL;

<?php
$rows = [
  ["city" => "Nairobi", "units" => 12],
  ["city" => "Mombasa", "units" => 8],
  ["city" => "Kisumu", "units" => 5],
];
$busy = array_filter($rows, fn($r) => $r["units"] >= 8);
echo implode(", ", array_map(fn($r) => $r["city"], $busy)), PHP_EOL;

<?php
$units = [12, 8, 5];
echo array_sum($units), PHP_EOL;

<?php
echo implode(",", array_keys(array_filter(["Nairobi" => 12, "Eldoret" => 3], fn($n) => $n >= 8))), PHP_EOL;