$collection = new PostCollection();

$collection[] = new Post(1);

$collection[] = 'abc';

foreach ($collection as $item) {
    echo $item->getId();
}

class PostCollection extends ArrayIterator
{
    public function current() : ?Post { /* … */ }

    public function offsetGet(mixed $key) : ?Post { /* … */ }

    public function offsetSet(mixed $key, mixed $value): void
    {
        if (! $value instanceof Post) {
            throw new InvalidArgumentException("…");
        }
        
        // …
    }
}

$posts = [];

$posts[] = 1;
$posts[] = 'string';

$posts = [];