Extract Function and Extract Class

Legacy PHP pages often mix SQL, business rules and HTML in one 1,000-line file. Refactor in tiny steps, running tests after each: extract function for a coherent block (give it parameters instead of relying on surrounding variables), then extract class when several functions share data. IDE automated refactorings (PhpStorm) and Rector reduce mistakes. Commit refactors separately from behavior changes.

<?php
// Before: inline in checkout.php
$total = 0;
foreach ($_SESSION['cart'] as $item) {
    $total += $item['price'] * $item['qty'];
}
if ($_POST['coupon'] === 'TEN') { $total = $total * 0.9; }

// After: extracted, typed and testable
final class CartTotals
{
    /** @param array<int, array{price:int, qty:int}> $items */
    public static function subtotal(array $items): int
    {
        return array_sum(array_map(fn(array $i) => $i['price'] * $i['qty'], $items));
    }

    public static function applyCoupon(int $subtotal, string $coupon): int
    {
        return $coupon === 'TEN' ? (int) round($subtotal * 0.9) : $subtotal;
    }
}

$total = CartTotals::applyCoupon(CartTotals::subtotal($_SESSION['cart']), (string) ($_POST['coupon'] ?? ''));

Common pitfalls

• Changing behavior (rounding!) while extracting — characterize first.
• Extracting functions that still read globals.
• Huge refactoring PRs that nobody can review.

Key points

• Small steps with tests after each
• Pass data as parameters
• Separate refactoring from behavior changes

Further reading: Refactoring catalog: Extract Function