PHP singleton mode example analysis, PHP singleton mode application scenarios

Single case mode (responsibility mode): An object (before learning design patterns, you need to understand object-oriented thinking) is only responsible for a specific task;

Single instance class: 1. The constructor needs to be marked as private (access control: to prevent external code from using the new operator to create objects). The singleton class cannot be instantiated in other classes and can only be instantiated by itself; 2. Have a static member variable that holds an instance of the class 3. Have a public static method to access this instance (the getInstance() method is commonly used to instantiate a singleton class, and the instanceof operator can be used to detect whether the class has been instantiated)