PHP Singleton development mode (responsibility mode)

Unlike normal classes, a singleton class cannot be directly instantiated in other classes. A singleton class can only be instantiated by itself. to obtain this restriction, you must mark the constructor as private. To make the singleton class take effect, you must provide an instance for other classes and use it to call various methods. As a model

Unlike normal classes, a singleton class cannot be directly instantiated in other classes. A singleton class can only be instantiated by itself. to obtain this restriction, you must mark the constructor as private. To make the singleton class take effect, you must provide an instance for other classes and use it to call various methods. As part of this mode, you must create an empty private _ clone () method to prevent objects from being copied and cloned. The Singleton is summarized as follows:
1. they must have a constructor marked as private.
2. they have a static member variable that saves the class instance.
3. they have a public static method to access this instance. The instance code is as follows:

Flat view printing?

01

ClassDB

02

{

03	Private $ _ db;

04

05	// Store the static variables of an instance

06	Privatestatic $ _ instance;

07

08	// Construct a private method

09	Privatefunction _ construct ()

10

{

11	$ This-> _ db = mysql_connect ('localhost', 'root', 'admin ');

12

}

13	// Prevent the object from being cloned

14	Privatefunction _ clone ()

15

{

16

}

17	PublicstaticfunctiongetInstance ()

18

{

19	// Prevent multiple instances

20	If (! (Self: $ _ instanceinstanceof self ))

21

{

22	Self: $ _ instance = newself ();

23

}

24	Returnself: $ _ instance;

25

}

26	// Other methods

27	Publicfunctionquery ()

28

{

29	// Code goes here

30	Return 'query ';

31

}

32	PublicfunctionselectDB ()

33

{

34	// Code goes here

35	Return 'selectdb ';

36

}

37

}

38

39	// Obtain the instance of the class

40	$ Object = DB: getInstance ();

41

42	// Access the method

43	Echo $ object-> query ();

This class begins to declare two variables, one is the private variable $ _ db storage database connection, will be filled during the construction, and the other is used for storage class instances. The next step is private constructor and magic function clone, which can prevent classes from being instantiated and cloned externally. the getInstance static method is used for the worker class instance. the code here can ensure the existence of a single instance, reduce system resource consumption and return instance references.