单例在 PHP 中的实现
Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.
Singleton has almost the same pros and cons as global variables. Although they're super-handy, they break the modularity of your code.
You can just use a class, which depends on Singleton, in some other context. You'll have to carry the Singleton class as well. Most of the time, this limitation comes up during the creation of unit tests.
在 PHP 中使用模式
Usage examples: A lot of developers consider the Singleton pattern an antipattern. That's why its usage is on the decline in PHP code.