<?php

$array = [
    'aaaa' => [
        'k1' => str_repeat('a', 1000),
    ]
];

switch ($argv[1] ?? 1) {
    default:
    case 1:
        $before = memory_get_usage();
        foreach ($array as &$element) {
            $element['k2'] = str_repeat('a', 1000);
            echo 'Ref in: ' . (memory_get_usage() - $before) . PHP_EOL; // 1328
        }
        echo 'Ref out: ' . (memory_get_usage() - $before) . PHP_EOL; // 1360
        break;

    case 2:
        $before = memory_get_usage();
        foreach ($array as $i => $element) {
            $array[$i]['k2'] = str_repeat('a', 1000);
            echo 'Direct in: ' . (memory_get_usage() - $before) . PHP_EOL; // 2032
        }
        echo 'Direct out: ' . (memory_get_usage() - $before) . PHP_EOL; // 1688
        break;

    case 3:
        $before = memory_get_usage();
        foreach ($array as $i => $element) {
            $element['k2'] = str_repeat('a', 1000);
            $array[$i] = $element;
            echo 'Get/Set in: ' . (memory_get_usage() - $before) . PHP_EOL; // 2032
        }
        echo 'Get/Set out: ' . (memory_get_usage() - $before) . PHP_EOL; // 1312
        break;
}