test.php

<?php
function benchmark(string $label, callable $callback): void {
    $start = microtime(true);
    $callback();
    $end = microtime(true);
    $duration = $end - $start;
    echo str_pad($label, 35) . ': ' . number_format($duration * 1000, 4) . " ms\n";
}

$array = range(1, 2000000); // adjust for load
$len = count($array);

// foreach / 2
benchmark('foreach / 2', function () use ($array) {
    $result = [];
    foreach ($array as $v) {
        $result[] = $v / 2;
    }
});

// array_map with / 2
benchmark('array_map / 2', function () use ($array) {
    $result = array_map(fn($v) => $v / 2, $array);
});

// for with / 2 (count inside loop)
benchmark('for / 2 (inline count)', function () use ($array) {
    $result = [];
    for ($i = 0; $i < count($array); $i++) {
        $result[] = $array[$i] / 2;
    }
});

// for with / 2 (cached count)
benchmark('for / 2 (cached count)', function () use ($array, $len) {
    $result = [];
    for ($i = 0; $i < $len; $i++) {
        $result[] = $array[$i] / 2;
    }
});

// foreach / 2 + range check
benchmark('foreach / 2 + range check', function () use ($array) {
    $result = [];
    foreach ($array as $v) {
        if ( $v <= 2000000 ) {
            $result[] = $v / 2;
        }
    }
});

// foreach * 0.5
benchmark('foreach * 0.5', function () use ($array) {
    $result = [];
    foreach ($array as $v) {
        $result[] = $v * 0.5;
    }
});

// array_map with * 0.5
benchmark('array_map * 0.5', function () use ($array) {
    $result = array_map(fn($v) => $v * 0.5, $array);
});

// for with * 0.5 (cached count)
benchmark('for * 0.5 (cached count)', function () use ($array, $len) {
    $result = [];
    for ($i = 0; $i < $len; $i++) {
        $result[] = $array[$i] * 0.5;
    }
});

// foreach * 0.5 + range check
benchmark('foreach * 0.5 + range check', function () use ($array) {
    $result = [];
    foreach ($array as $v) {
        if ( $v <= 2000000 ) {
            $result[] = $v * 0.5;
        }
    }
});