Understanding PHP classes and objects: Basics of object-oriented programming

PHP is an open source server-side scripting language that can be embedded in HTML and create websites using standard PHP syntax or mixed HTML and PHP code.

Object-oriented programming (OOP) is a paradigm of modern programming languages, and PHP is one of the languages ​​that supports OOP. OOP programming can help developers better organize and maintain complex applications and improve code reusability and scalability.

In PHP, classes and objects are the two core concepts of object-oriented programming. This article will introduce classes and objects in PHP and provide some basic examples to help beginners get started.

Definition of class

In PHP, a class is a data type that describes a set of related properties and methods that can be used to create objects. A class is defined using the keyword class, followed by the class name, which can be named using camel case or underscore separation. The following code:

class MyClass {
    // 类的属性和方法
}

Classes can contain properties and methods. Properties are variables of a class that store state information about the class. Methods are functions of a class that implement the behavior of the class.

Definition of attributes

The attributes of a class can contain public attributes and private attributes. Public properties can be accessed from outside the class, while private properties can only be accessed from within the class.

Attributes are defined using the keywords public, private or protected, followed by the attribute name. The following code:

class Car {
    public $color;
    private $price;
    protected $engine;
}

The above code defines a class named Car, which contains a public attribute color and a private attribute price, as well as a protected attribute engine.

Definition of methods

Class methods can also contain public methods and private methods. Public methods can be accessed from outside the class, while private methods can only be accessed from within the class.

The definition of a method uses the keywords public, private or protected, followed by the method name and parentheses. The following code:

class Car {
    public function start() {
        // 启动引擎的代码
    }
    
    private function stop() {
        // 关闭引擎的代码
    }
    
    protected function brake() {
        // 刹车的代码
    }
}

The above code defines a class named Car, which contains a public method start and a private method stop, and also contains a protected method brake.

Creation of objects

An object is an instance of a class, which contains the specific implementation of the attributes and methods of the class.

In PHP, to create an object, you need to use the new keyword and pass in the class name and parameters. The parameters are optional. The following code:

$myCar = new Car();

The above code creates an object named myCar, which is an instance of the Car class.

Accessing properties and methods

The properties and methods of a class can be accessed using the dot operator (.). The following code:

$myCar->color = '红色';    // 设置属性color为红色
$myCar->start();           // 调用方法start

The above code sets the value of the object's attribute color to red, and calls the object's method start.

Inheritance of classes

In PHP, classes can inherit the properties and methods of other classes, so that more complex class hierarchies can be built.

To inherit from other classes, you need to use the keyword extends and the class name of the parent class. The following code:

class SUV extends Car {
    // SUV类的属性和方法
}

The above code defines a class named SUV, which inherits all properties and methods of the Car class.

Summary

PHP is a language that supports object-oriented programming. Classes and objects are the two core concepts of object-oriented programming. Classes can contain properties and methods, and objects are instances of classes.

The properties and methods of a class can use the keywords public, private or protected to define access levels. The properties and methods of a class whose access control is public can be accessed outside the class, private properties and methods can only be accessed inside the class, and protected properties and methods can be accessed within the class and its subclasses.

Through inheritance, classes can inherit the properties and methods of other classes, and more complex class hierarchies can be built.

For beginners of PHP, it is very important to master the basic knowledge of classes and objects. Only by practicing continuously in practice and deeply understanding the principles and methods of OOP programming can you write high-quality PHP code.

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