(PHP 5 >= 5.1.0, PHP 7, PHP 8)
SplObserver::update — Receive update from subject
$subject
This method is called when any SplSubject to which the observer is attached calls SplSubject::notify().
本函数还未编写文档，仅有参数列表。
subject
The SplSubject notifying the observer of an update.
没有返回值。
