... in loop

spread_in_loop.php

<?php

function printMemory($start, $header) {
    printf(
        "%s - Time: %s | Memory (current): %s KB | Memory (max): %s KB" . PHP_EOL,
        $header,
        number_format(round((microtime(true) - $start) * 1000, 6), 6, ',', ''),
        number_format(round((memory_get_usage() / 1024), 2), 2, ',', ''),
        number_format(round((memory_get_peak_usage() / 1024), 2), 2, ',', '')
    );
}

$countProducts = 500;
$countTags = 1;

$products = [];
for($i = 1; $i <= $countProducts; $i++) {
    $tags = [];
    for($n = 1; $n <= $countTags; $n++) {
        $tags[] = 'tag_' . $i . '_' . $n;
    }
    
    $products[] = [
        'id' => $i,
        'tags' => $tags,
    ];
}

$start = microtime(true);
$tags = [];
foreach($products as $product) {
    $tags[] = $product['tags'];
}
$tags = array_merge(...$tags);
printMemory($start, '... in loop');