Detailed explanation of the role and usage of the extends keyword in PHP

Detailed explanation of the role and usage of the extends keyword in PHP

In PHP programming, extends is a very important keyword, which is used to implement class inheritance. Through the extends keyword, we can create a new class that can inherit the properties and methods of one or more existing classes. Inheritance is an important concept in object-oriented programming, which makes code reuse and extension more convenient and flexible. This article will introduce in detail the function and use of the extends keyword.

1. The role of the extends keyword
   The role of the extends keyword is to implement class inheritance. When a class inherits another class, it needs to use the extends keyword. Through inheritance, subclasses can obtain the properties and methods of the parent class, and can extend and rewrite them on this basis. Inheritance can also help us reuse code and avoid writing the same code repeatedly.
2. How to use the extends keyword
   The method of using the extends keyword is very simple. When defining a class, you only need to add the extends keyword and the name of the parent class after the class. Here is an example:

class ParentClass {
   // 父类的属性和方法
}

class ChildClass extends ParentClass {
   // 子类的属性和方法
}

In the above example, the ChildClass class inherits the ParentClass class. After inheritance, ChildClass can use the properties and methods of ParentClass.

3. Inherited access control
   In inheritance, the inheritance relationship has three access control modifiers: public, protected and private.

• public: Properties and methods declared as public remain public after inheritance and can be accessed by subclasses.
• protected: Properties and methods declared as protected become protected after inheritance and can only be accessed in the parent class and subclass, and cannot be accessed outside the class.
• private: Properties and methods declared as private cannot be accessed by subclasses after inheritance.

Inherited access control modifiers can be specified by adding the corresponding modifiers in front of properties and methods.

4. Extend and rewrite the methods of the parent class
   In the subclass, we can extend and rewrite the methods of the parent class. If a method with the same name as the parent class is defined in the subclass, the method in the subclass will override the method in the parent class. However, we can also call overridden methods in the parent class by using the parent keyword.

The following is an example:

class ParentClass {
   public function sayHello() {
      echo "Hello, I am the parent class.";
   }
}

class ChildClass extends ParentClass {
   public function sayHello() {
      echo "Hello, I am the child class.";
   }
  
   public function sayParentHello() {
      parent::sayHello();
   }
}

In the above example, ChildClass overrides the sayHello() method of ParentClass and adds a new method sayParentHello(). In the sayParentHello() method, the parent class's sayHello() method is called through parent::sayHello().

5. Using multiple inheritance
   PHP supports multiple inheritance, that is, a subclass can inherit multiple parent classes. When using the extends keyword, you can specify multiple parent classes at the same time, separated by commas. However, it should be noted that PHP does not support multiple inheritance with the same method name.

The following is an example:

class ParentClass1 {
   // 父类1的属性和方法
}

class ParentClass2 {
   // 父类2的属性和方法
}

class ChildClass extends ParentClass1, ParentClass2 {
   // 子类的属性和方法
}

In the above example, ChildClass inherits both ParentClass1 and ParentClass2.

Through the introduction of this article, we understand the role and use of the extends keyword in PHP. The extends keyword makes class inheritance very simple and can help us achieve code reuse and extension. Mastering the use of the extends keyword is very important for PHP programming. I hope this article can be helpful to everyone.

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