Avoiding Repeated Work

<?php
$prices = [100, 250, 400];
$taxRate = 0.18;

// Repeats the same calculation on every pass
$slow = [];
foreach ($prices as $price) {
    $multiplier = 1 + (18 / 100);
    $slow[] = $price * $multiplier;
}

// Computed once
$multiplier = 1 + $taxRate;
$fast = array_map(fn($p) => $p * $multiplier, $prices);

echo $slow === $fast ? "same result" : "different";

same result

<?php
function fibonacci(int $n): int
{
    static $memo = [0 => 0, 1 => 1];

    return $memo[$n] ??= fibonacci($n - 1) + fibonacci($n - 2);
}

echo fibonacci(50);

12586269025

<?php
$orders = [["id" => 1, "user" => 7], ["id" => 2, "user" => 9], ["id" => 3, "user" => 7]];
$users = [["id" => 7, "name" => "Ana"], ["id" => 9, "name" => "Ben"]];

// Slow idea: search the users list for every order (a loop inside a loop)
// Fast idea: index the users by id once
$byId = array_column($users, "name", "id");

foreach ($orders as $order) {
    echo "Order {$order['id']}: {$byId[$order['user']]}\n";
}

Order 1: Ana
Order 2: Ben
Order 3: Ana

<?php
$pdo = require __DIR__ . "/db.php";

$posts = $pdo->query("SELECT id, name AS title, category FROM products")->fetchAll();

$queries = 1;

// N+1: a query for each product's category count
foreach ($posts as $p) {
    $stmt = $pdo->prepare("SELECT COUNT(*) FROM products WHERE category = ?");
    $stmt->execute([$p["category"]]);
    $stmt->fetchColumn();
    $queries++;
}

echo "N+1 style: $queries queries\n";

// Better: ONE query for all the counts
$counts = $pdo->query("SELECT category, COUNT(*) AS n FROM products GROUP BY category")->fetchAll(PDO::FETCH_KEY_PAIR);

echo "Grouped: 2 queries in total, categories: " . count($counts);

N+1 style: 6 queries
Grouped: 2 queries in total, categories: 3