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$book = ['The title', 'the author'];
// Old version
list($title, $author) = $book;
// In new version
[$title, $author] = $book;

// Complex array.
$books = [
    ['title' => 'Chernovik', 'author' => 'Lukjznenko'],
    ['title' => 'Dnevnow Dozor', 'author' => 'Lukjznenko'],
];

['title' => $book, 'author' => $author] = $books[0];

foreach ($books as ['title' => $book, 'author' => $author]) {
    var_dump($title, $author);
}

class User{
    protected $age;

    public public function age() : ?int
    {
        return $this->age;
    }

    public function subscribe(?callable $callback) {
        
        var_dump('subcribe here');
        $callback();        
    }
}

$age = (new User)->age();
var_dump($age);

(new User)->subscribe(function() {
    var_dump('respond here');
});
//or
(new User)->subscribe(null);