Types & strict mode

PHP will happily turn "2" into 2 if you let it. I've decided not to let it.

The one line that matters

Put this at the top of every PHP file you write:

<?php

declare(strict_types=1);

It applies per file. Call a strict file from a non-strict one and coercions can still sneak in at the boundary - so be consistent.

Scalar types

<?php

declare(strict_types=1);

function demo(
    int $i,
    float $f,
    string $s,
    bool $b,
): void {
    // ...
}

void means "returns nothing useful". There's also never for functions that always throw or exit.

Nullable & unions

<?php

declare(strict_types=1);

function findName(?string $id): string|null
{
    if ($id === null) {
        return null;
    }

    return 'Lukasz';
}

// same idea:
function findName2(?string $id): ?string
{
    return $id === null ? null : 'Lukasz';
}

Unions (string|int) and intersections (Countable&Traversable) exist. Keep them readable - if a union grows wild, maybe you want a class or enum.

Mixed, iterable, object, array

Type	Meaning
mixed	Anything - escape hatch, use sparingly
array	PHP array (list or map)
object	Any object
iterable	array or Traversable
callable	Something you can call

Return types & properties

<?php

declare(strict_types=1);

class Counter
{
    private int $value = 0;

    public function bump(): int
    {
        return ++$this->value;
    }
}

Typed properties are normal now. Uninitialized typed properties throw if you read them too early - good.

Type juggling without strict_types (why we don't)

<?php

// NO declare(strict_types=1) - bad demo only

function add(int $a, int $b): int
{
    return $a + $b;
}

add(1, '2'); // becomes add(1, 2) - silent coercion

With strict types, that's a TypeError. Prefer the error.

Casting when you mean it

<?php

declare(strict_types=1);

$id = (int) '42';
$label = (string) 42;
$flag = (bool) 1;

Explicit casts are fine when data comes from HTTP/query strings (everything is a string out there). Validate, then cast.

instanceof & get_debug_type

<?php

declare(strict_types=1);

function describe(mixed $value): string
{
    if ($value instanceof DateTimeImmutable) {
        return 'datetime';
    }

    return get_debug_type($value);
}

get_debug_type() is nicer than gettype() for error messages.

Practical rule

• Always strict_types
• Type parameters, returns, and properties
• Use ?Type / unions thoughtfully
• Cast at the HTTP/DB edge, not deep in domain code

Run examples/types/types.php once you've got PHP installed - it's a tiny playground for this chapter.

Best,

-- ł.