In-depth understanding of PHP object-oriented programming: inheritance of object properties and methods

PHP Property and method inheritance in object-oriented programming: Subclasses inherit the non-private properties of the parent class, which can be modified through the parent keyword. Subclasses inherit the non-private methods of the parent class and can be overridden through override. PHP supports multi-level inheritance, but methods with the same name only inherit the methods of one of the parent classes.

In-depth understanding of object-oriented programming in PHP: inheritance of object properties and methods

In PHP, the inheritance of object properties and methods Inheritance is a key concept in object-oriented programming. It allows subclasses to access and modify the properties and methods of their parent class.

Attribute inheritance

When a subclass inherits from a parent class, the subclass will automatically inherit all non-private properties of the parent class. Subclasses can use these properties but cannot modify them directly. To modify the properties of a parent class, the child class must use the parent:: keyword. For example:

class ParentClass {
    protected $name = 'Parent';
}

class ChildClass extends ParentClass {
    public function getName() {
        return parent::$name;
    }
}

$child = new ChildClass();
echo $child->getName(); // Output: Parent
$child->name = 'Child'; // Error: Cannot modify inherited property directly

Method inheritance

The subclass also inherits all non-private methods of the parent class. Subclasses can use these methods, or they can use the override keyword to redefine methods in the parent class. For example:

class ParentClass {
    public function sayHello() {
        echo 'Hello from Parent!';
    }
}

class ChildClass extends ParentClass {
    public function sayHello() {
        parent::sayHello(); // Call the parent method
        echo 'Hello from Child!';
    }
}

$child = new ChildClass();
$child->sayHello(); // Output: Hello from Parent! Hello from Child!

Multi-level inheritance

PHP supports multi-level inheritance, which means that a child class can inherit from multiple parent classes. In this case, the child class will inherit all the properties and methods of the parent class. However, if multiple parent classes define methods with the same name, the child class will only inherit the method from one of the parent classes. To access the methods of another parent class, the subclass must use the parent:: keyword.

Practical case:

Library management system

Consider a library management system with Book and EBook Class:

class Book {
    private $title;
    private $author;

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