Detailed explanation of inherited access type control of classes in PHP objects

Type of Access control Allows developers to restrict access to members in a class by using modifiers. This is a new feature of PHP5 and an important feature in OOP languages. Most OOP languages ​​already support this feature. PHP5 supports the following three access modifiers, two of which we have introduced in class encapsulation. To summarize here, Access control modifiers include public (public, default), private (private) and protected (protected). Their functions and the differences between them are shown in the following table:

①Public access modifier public

Use this With this modifier, the members in the class will have no access restrictions, and all external members can access the members in this class. In all versions before PHP5, the members of the class in PHP are public.

②Private access modifier private

When a member of a class is defined as private, there are no access restrictions for all members of the same class, but for the External code of the class is not allowed to change or even operate, and subclasses of the class cannot access private-modified members. The code is as follows:

<?php
    //声明一个类作为父类使用，将它的成员都声明为私有的
    class MyClass {
        private $var1=100;
 
        private function  printHello(){
            echo "hello<br>";     
        }
    }
 
    //声明一个Myclass类的子类师徒访问父类中的私有成员
    class MyClass2 extends MyClass{
        function useProperty(){
            echo "输出从父类继承过来的成员属性值".$this ->var1."<br>";
            $this ->printHello();
        }
    }
 
    $subObj = new MyClass2();
    $subObj -> useProperty();       //调用子类对象中的方法实现对父类私有成员的访问
?>

In the above code, a class MyClass is declared, a private member attribute and a private member method are declared in the class, and A class MyClass2 is declared to inherit the class MyClass, and the private members in the parent class are accessed in the subclass MyClass2. But the private members in the parent class can only be used in its own class, so the access error occurs.

③Protected access modifier protected

Members modified as protected have access to subclasses of this class and subclasses of subclasses Permissions allow you to read and write properties and methods. But it cannot be accessed by external code of this class, and external code of this subclass does not have additional permissions to its properties and methods. By changing the access rights of the parent class in the above example to protected, the members of the parent class can be accessed in the subclass, but they cannot be accessed outside the class, so the purpose of encapsulating the object can also be completed. The code is as follows:

<?php
    class MyClass {
        protected $var1=100;
 
        protected function  printHello(){
            echo "hello<br>";     
        }
    }
 
    //声明一个Myclass类的子类师徒访问父类中的保护成员
    class MyClass2 extends MyClass{
        function useProperty(){
            echo "输出从父类继承过来的成员属性值".$this ->var1."<br>";
            $this ->printHello();
        }
    }
 
    $subObj = new MyClass2();
    $subObj -> useProperty();       //调用子类对象中的方法实现对父类私有成员的访问
 
    echo $subObj ->var1;         //试图访问类中受保护的成员，结果出错
?>

In the above example, the members in the class MyClass are set to protected using the protected modifier, and they can be accessed directly in the subclass. But it is an error to access protected modified members outside the subclass.

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