static and const in php

The static keyword describes that member properties and member methods in a class are static; what are the benefits of static members? Earlier we declared the human being "Person". If we add an attribute of "the country to which the person belongs" to the "Person" class, hundreds or more instance objects can be instantiated using the "Person" class. Each object has the attribute of "country to which it belongs." If the project is developed for Chinese people, then each object will have the attribute of a country "China" and other attributes are different. If we make the "country" attribute a static member, so that there is only one country attribute in the memory, and let hundreds or more objects share this attribute, the static member can restrict external access, because Static members belong to the class and do not belong to any object instance. They are allocated space when the class is loaded for the first time. They are inaccessible to other classes and are only shared with instances of the class. The member of the class can be accessed to a certain extent. Form protection;

Let’s analyze it from the perspective of memory. The memory is logically divided into four segments. The objects are placed in the “heap memory”, the references of the objects are placed in the “stack memory”, and the static members It is placed in the "Initialization Static Segment", which is placed when the class is loaded for the first time, so that it can be shared by every object in the heap memory, as shown below:

The static variables of the class are very similar. Global variables can be shared by all instances of a class, as can static methods of a class, similar to global functions.

classPerson

01

{

03

04

static$myCountry

05

public

=

"中国";

06

07// var $name; //The person’s name

08

09//This is a human static member method

10

say() {

11	“I am Chinese”echo"I am Chinese"; 12

​​​

}

13}

14

15

//Output static attributes

16$myCountry

echo

Person::

;

17

18//Access static method

19

Person:: say();

20

21

//Reassign the static attribute

22=

Person::

$myCountry

"USA"

;

23

echo

Person: :

$myCountry;

24

?>

Because static members are created when the class is loaded for the first time, you can access the static members by using the class name without requiring an object outside the class; as mentioned above, static members are Shared by each instance object of a class, can we access static members in the class using objects? From the picture above, we can see that static members do not exist inside each object, but each object can be shared, so if we use objects to access members, there will be no such attribute definition, Using object access does not Regarding static members, in other object-oriented languages, such as Java, you can use objects to access static members. If you can use objects to access static members in PHP, we should try not to use them because static members When we are working on a project, our purpose is to use the class name to access it.

Static methods in a class can only access the static attributes of the class

. Static methods in the class cannot access non-static members of the class. The reason is very simple. We want to access this class in the method of this class. For other members, we need to use the $this reference, and the $this reference pointer represents the object that calls this method. We have said that static methods are not called with objects, but are accessed using class names, so it is basically There will be no object, and there will be no reference to $this. Without the reference to $this, non-static members in the class cannot be accessed, and because static members in the class can be used without objects. to access, so the static methods in the class can only access the static properties of the class. Since $this does not exist, we use a special class "self" to access other static members in the static method; self and $this is similar, except that self represents the class where this static method is located. So in a static method, you can use the "class name" of the class where the method is located, or you can use "self" to access other static members. If there are no special circumstances, we usually use the latter, that is, "self: :Member attribute" method.

01

class

Person

03

{

0405

      static"中国";

public

$myCountry=

07

06

//This is a human static member method, access other static members through self 08

public function

static

say() {

09                                                                                                  ry10

;

}

11}

12

13

//Access static method

14

Person::say();

15?> Use the class name or "self::member attribute form".

const is a keyword for defining constants. To define constants in PHP, you use the "define()" function, but to define constants in a class, you use the "const" keyword, which is similar to #define in C. If its value is changed in the program, an error will occur. The member attributes modified with "const" are accessed in the same way as the members modified with "static". They also use "class name" and use " self" keyword. But you don't need to use the "$" symbol, and you can't use objects to access it.

01

</p><p><table><tbody><tr><td><code>02

classMyClass

03

{

04	//Define a constant

05	'constant value';

06

07

function

showConstant() {

08

echo	self::constant . " "; // Use self to access, do not add "$"

09

}

10

11

12echo

MyClass::constant .

" ";

//Use the class name to access without adding "$"

13

14$class

=

newMyClass();$class

15

->showConstant();

// echo $class::constant; is not allowed

16

?>

17

The above introduces static and const in PHP, including related content. I hope it will be helpful to friends who are interested in PHP tutorials.