Let's talk about the usage of new keyword in PHP

PHP is a high-level programming language widely used in the fields of web development and server-side programming. In PHP, new is a very important keyword, used to create instances of classes. This article will introduce the usage of the new keyword in PHP and its related concepts.

1. What is a class

In object-oriented programming (OOP), a class (Class) is a data type that defines the properties and methods of an object. A class can be seen as a template for an object, which describes what properties and behaviors the object should have. Classes are the foundation of object-oriented programming and implement core concepts such as data encapsulation, inheritance, and polymorphism.

2. What is an object?

An object (Object) is an instance of a class. The process of creating an object is called instantiation, which uses a class to create a specific instance. Objects can call methods and properties of the class, and can also change their own property values ​​as needed. In PHP, objects are usually instantiated through the new keyword.

3. Usage of new keyword

In PHP, use the new keyword to create an instance of a class. The syntax format of the new keyword is as follows:

$object = new ClassName();

Among them, $object is the variable name, which can be any legal variable name, and ClassName is the class name, which needs to be specified after the keyword new. If the class is defined in the namespace, you need to specify the complete namespace path, such as:

$object = new Namespace\ClassName();

When using the new keyword to instantiate an object, you must pay attention to the following points:

• Class The name, method and attribute names of the class are all case-sensitive;
• The class name after the new keyword must be a legal class name, otherwise a syntax error will occur;
• If the class There is no constructor defined and the parentheses can be omitted.

4. Constructor

The constructor is a special type of method that is automatically called when an object is created. Constructors are typically used to initialize an object's properties or perform other necessary operations. In PHP, constructors have the same name as the class and they must be declared public, otherwise they cannot be accessed from outside the class. The usage of the constructor is as follows:

class ClassName {
    public function __construct() {
        // 构造函数的代码
    }
}

When using the new keyword to instantiate an object, the constructor will be automatically called. If you do not need to perform any operations in the constructor, you can omit the definition of the __construct() method.

5. Destructor

The destructor is a function used to destroy objects. The destructor is automatically called when an object goes out of scope or is explicitly destroyed. In PHP, the name of the destructor is __destruct(), and its usage is as follows:

class ClassName {
    public function __destruct() {
        // 析构函数的代码
    }
}

If you do not need to perform any operations in the destructor, you can omit the definition of the __destruct() method.

6. Class attributes and methods

The attributes of a class (Property) are variables defined in the class. A class method is a function defined in the class. The access control characters of properties and methods can be one of three types: public, private, or protected, which represent public, private, and protected members respectively.

Public members refer to members that can be accessed from inside the class, subclasses, and outside the class.

Private members refer to members that can only be accessed within the class.

Protected members are members that can only be accessed within the class and in subclasses.

Use the keywords var, public, private, and protected to declare the attributes of the class, and use the function keyword to declare the methods of the class.

The following is an example of a class that demonstrates the usage of properties and methods:

class Person {
    var $name;      // 公共属性
    private $age;   // 私有属性

    public function __construct($name, $age) {
        $this->name = $name;
        $this->age = $age;
    }

    public function say() {   // 公共方法
        echo "My name is {$this->name}, I'm {$this->age} years old.";
    }

    private function secret() {  // 私有方法
        echo "This is a secret method.";
    }
}

In the above code, the Person class has two properties: $name and $age. Among them, $name is a public property that can be accessed from inside the class, subclasses, and outside the class, while $age is a private property that can only be accessed from within the class. The Person class has a constructor __construct(), a public method say() and a private method secret(). The constructor is used to initialize the $name and $age properties, the say() method is used to output information about the Person object, and the secret() method can only be called inside the class and cannot be accessed from outside the class.

7. Summary

The new keyword is an important way to create an instance of a class in PHP. It cooperates with the constructor defined in the class to initialize the object. Class properties and methods can be public, private, and protected, with different roles and access rights. In PHP, although the concepts of classes and objects are relatively abstract, understanding and proficiently using these keywords and concepts can help develop more efficient, stable, and scalable Web applications.

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