You must understand what abstract classes and abstract methods are in PHP

In the previous article, I brought you "Five minutes to show you the interface declaration and application in PHP (detailed examples)", which introduced the declaration and application issues of the PHP interface in detail , in this article we take a look at abstract classes and abstract methods in PHP, I hope it will be helpful to everyone!

In the previous article, I introduced the interface, which mentioned that the interface can be understood as a special abstract class. So what is an abstract class? If you want to know what Abstract classes We first need to talk about what abstract methods are. The emergence of abstract classes and abstract methods is mainly to facilitate class inheritance. Then let’s take a look at what abstract methods and abstract classes are.

What is an abstract method

In our previous study, a class can have multiple subclasses, we define abstract methods It is to facilitate class inheritance. The methods defined in our class without a method body are abstract methods. So what is a method body? The method body refers to the curly brackets and the content within the curly brackets when the class is declared. The absence of a method body means that there is no such content. Instead, it is directly placed after the method statement and ends with a semicolon during the declaration. This is called an abstract method.

It should be noted that abstract methods must be modified with the keyword "abstract" when declaring them. Its syntax format is as follows:

abstract 访问权限修饰符 function 方法名1(参数列表);
abstract 访问权限修饰符 function 方法名2(参数列表);

The understanding of abstract methods is relatively simple. Next, let’s take a look at what an abstract class is.

What is an abstract class

#Through the above we already know what an abstract method is, so the definition of an abstract class is actually very It's easy to understand. When a method in a class is an abstract method, we define this class as an abstract class. At the same time, abstract classes also need to be modified using the "abstract" keyword. In abstract classes, there can be member attributes that are not abstract methods, but once a method is an abstract method, then this class is an abstract class.

It should be noted that not all member methods of abstract classes are abstract methods. Those member methods and attribute access permissions that are not abstract methods cannot be private. The same is true for methods in abstract classes. Needs to be inherited by subclasses.

Let me give you an example:

abstract class demo{
var $test;
abstract function fun1();
abstract function fun2();
}

Characteristics of abstract classes

• Abstract classes Both abstract methods need to be modified with abstract;

• Abstract methods must be in abstract classes, and there can be no abstract methods in abstract classes;

• Abstract Classes cannot directly create objects (instantiate);

• If a subclass of an abstract class overrides all abstract methods, it is a concrete class; a subclass of an abstract class does not override all Abstract method, then the subclass is still an abstract class;

The abstract class contains abstract methods that cannot be implemented, which means that the abstract class cannot be instantiated, then the object cannot be created, so We can't use it directly. So since we can't use it directly, what problems can we solve by using abstract classes?

Next let’s look at it through an example, define an abstract class, and then use another class to inherit this abstract class and implement the abstract methods in the abstract class.

The example is as follows:

<?php
    abstract class father{
        public $study = &#39;好好学习<br>&#39;;
        public $study1 = &#39;天天向上<br>&#39;;
        abstract function title();
        abstract function output();
    }
    class Demo extends father{
        public function title(){
            echo &#39;福如东海，寿比南山。&#39;;
        }
        public function output(){
            echo $this -> study.$this -> study1;
        }
    }
    $obj = new Demo();
    $obj -> output();
    $obj -> title();
?>

Output result:

From the above example, we implement the definition of an abstract class, and then Use another class to inherit this abstract class and implement the abstract methods in the abstract class.

As mentioned above, the previous article talked about the relevant knowledge of PHP interface. It said that the interface can be understood as a special abstract class. So since it is special, what is the difference between PHP interface and abstract class? What about the similarities? Let’s take a look below. It is recommended that you first read "Five minutes to show you the interface declaration and application in PHP (detailed examples)" which introduces the PHP interface.

The difference between PHP interfaces and abstract classes

• Abstract classes can have attributes, ordinary methods, and abstract methods, but interfaces cannot have attributes, ordinary methods Methods can have constants.

• There may not be abstract methods in abstract classes, but there must be "abstract" methods in interfaces.

• Abstract classes are declared before the class with the abstract keyword, and class is declared as a class. The interface is declared with interface, but cannot be declared with class, because the interface is not a class.

• Abstract classes use the extends keyword to allow the subclass to inherit the parent class and then implement detailed abstract methods in the subclass. The interface uses implements to allow ordinary classes to implement detailed methods of the interface in the class, and the interface can implement multiple methods at one time, just use commas to separate each interface.

The similarities between PHP interface and abstract class

• are all used to declare a certain thing, standardize names and parameters, and form modules. There are no detailed implementation details.

• All related details are implemented through classes.

• Syntactically, the abstract method of an abstract class is the same as an interface and cannot have a method body, that is, the {} symbol.

• You can use inheritance. Interfaces can inherit interfaces to form new interfaces, and abstract classes can inherit abstract classes to form new abstract classes.

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