Performance of array_merge comparing to normal element addition in a loop

benchmark_array_merge_in_loop.php

<?php

$mergeStrategy = $argv[1];
$numberOfNewElements = $argv[2];

$isArrayMergeStrategy = $mergeStrategy === 'array_merge';
$isUnionStrategy = $mergeStrategy === 'union';

$list = [
    [0],
    [1],
];

$startMicrotime = microtime(true);

for ($i = count($list); $i < $numberOfNewElements; $i++) {
    if ($isArrayMergeStrategy) {
        $list = array_merge($list, [$i => [$i]]);

        continue;
    }

    if ($isUnionStrategy) {
        $list[$i] = [$i];

        continue;
    }
}

$endMicrotime = microtime(true);

echo sprintf(
    "Building array of %d elements using %s took %d seconds.\n",
    count($list),
    $mergeStrategy,
    $endMicrotime - $startMicrotime
);

echo sprintf(
    "Memory usage is: %d MB.\n",
    memory_get_usage() / 1024 / 1024
);

echo sprintf(
    "Memory peak was: %d MB.\n",
    memory_get_peak_usage(true) / 1024
);