为什么在语句中需要赋值操作符?

我正在尝试一些不同的设计模式来学习它们，并开始使用DI容器。

主代码(index.php)

$container = new 'League'Container'Container();
$container->add("config", function(){
    return new Config(APP_ROOT . "/config.json");
});

$container->add("GoogleBooks", GoogleBooksProvider::class)
    ->withArgument( $container['config'] );

$container->add("books", BookRepository::class);
// empty array, as expected
var_dump($container['books']->getProviders());
// this line doesn't add the provider
$container['books']->addProvider( $container['GoogleBooks'] );
// empty array, should expect to have one entry, GoogleBooksProvider
var_dump($container['books']->getProviders());

BookRepository: addProvider

public function addProvider( iProvider $provider ) {
    $this->_providers->push($provider);
    return $this;
}

不像预期的那样工作，代码注释中描述的问题。但是，如果我交换

$container['books']->addProvider( $container['GoogleBooks'] );

$container['books'] = $container['books']->addProvider( $container['GoogleBooks'] );

通过将GoogleBooksProvider存储在BookRepository中，它可以正确工作。为什么我需要一个赋值运算符使其正确工作?

如果不将其放入容器中，则不需要赋值操作符，它将按预期工作。

$br = new BookRepository();
$br->addProvider( new GoogleBooksProvider($container['config']) );
// shows GoogleBooks is in the _providers array
var_dump($br->getProviders());