PHP 7.4: Typed Properties, Arrow Functions and Preloading

PHP 7.4 (2019) introduced typed properties, arrow functions (fn($x) => $x * 2), the spread operator in arrays, numeric literal separators (1_000_000), ??=, covariant returns/contravariant parameters, OPcache preloading and FFI. It deprecated nested ternaries without parentheses and curly-brace string offsets. Many legacy apps stopped at 7.4, which makes it the usual starting point for PHP 8 upgrades.

<?php
final class Invoice
{
    public int $id;
    public ?string $notes = null;
    /** @var LineItem[] */
    private array $items = [];

    public function total(): int
    {
        return array_sum(array_map(fn(LineItem $i) => $i->priceCents * $i->qty, $this->items));
    }
}

$all = [...$defaults, ...$overrides];   // spread in arrays (integer keys only in 7.4)
$limit = 10_000;

Common pitfalls

• Accessing a typed property before initialization throws Error: must not be accessed before initialization.
• Spreading arrays with string keys fails in 7.4 (allowed from 8.1).
• Unparenthesized nested ternaries — deprecated in 7.4 and a fatal error in 8.0.

Key points

• Typed properties and arrow functions
• Uninitialized typed properties throw
• 7.4 is the usual PHP 8 upgrade baseline

Further reading: PHP manual: PHP 7.4 new features