public public: this class, subclasses, and external objects can all call
protected: subclasses of this class can execute, external objects cannot call
private: only this class is executed, and neither subclasses nor external objects can be called
three major characteristics of object-oriented programming

1) closure

closeness can also be called information hiding . it is to separate the use and implementation of a class, leaving only limited interfaces (methods) to connect with the outside. for developers who use this class, they only need to know how to use this class, and do not need to care about how this class is implemented. doing so allows developers to better focus on other things, while also avoiding the inconvenience caused by interdependencies between programs.

2) inheritance

inheritance means that a derived class (subclass) automatically inherits the properties and methods in one or more base classes (parent classes), and can override or add new properties and methods. inheritance simplifies the creation of objects and classes and increases code reproducibility. inheritance is divided into single inheritance and multiple inheritance. php supports single inheritance, that is to say, a subclass has and has only one parent class.

3) polymorphism

polymorphism means that different objects of the same class can obtain different results using the same method. this technology is called polymorphism. polymorphism enhances software flexibility and reusability.

class definition

a class can contain its own constants, variables (called "properties") and functions (called "methods").
like many object-oriented languages, php also defines classes through the class keyword plus the class name. the format of the class is as follows:

the code is as follows:

<?php
??class?myobject{
????//……
????}
?>

definition: birds of a feather flock together, and objects with similar characteristics are grouped into a class. the class defines these similar objects. has the same properties and methods. a class is a description of similar objects, called the definition of the class, which is the blueprint or prototype of the object of that class.

an object of a class is called an instance of the class. to create an instance of a class, the new keyword must be used.

<?php
//類的定義以關鍵字class開始，類的命名通常以每個單詞第一個字母大寫
????class?nbaplayer{
????????public?$name?=?"jordan";?//定義屬性
????????public?$height?=?"198cm";
????????public?$team?=?"bull";
????????public?$playernumber?=?"23";
????????//定義方法
????public?function?run(){
????????echo?"running\n";
????}
????public?function?dribblr(){
????????echo?"dribbling\n";
????}
????public?function?pass(){
????????echo?"passing\n";
????}
}
????//類到對象的實例化
????//類的實例化為對象時使用關鍵字new，new之后緊跟類的名稱和一對括號
????$jordan?=?new?nbaplayer();?
????//對象中的屬性成員可以通過"->"符號來訪問
????echo?$jordan->name."\n";
????//對象中的成員方法可以通過"->"符號來訪問
????$jordan->dribble();
????$jordan->run();
?>

member methods

functions in a class are called member methods. the only difference between functions and member methods is that functions implement an independent function, while member methods implement a behavior in the class and are part of the class.
let’s expand the myobject class above and add a member method to it. the code is as follows:

<?php
classmyobject{
???function?getobjectname($name){
??????echo?"商品名稱為：".$name;
???}
}
?>

the function of this method is to output the product name, which is passed in through the parameters of the method. .
a class is an abstract description, a collection of objects with similar functions. if you want to use the methods and variables in the class, you must first implement them into an entity, that is, an object.

class constants
since there are variables, of course there are also constants. a constant is a quantity that does not change and is a constant value. a well-known constant is pi. to define constants, use the keyword const, such as:
constpi=3.14159;

constructor

php 5 allows developers to define a method as a constructor in a class. classes with a constructor will call this method every time a new object is created, so it is very suitable for doing some initialization work before using the object.

destructor

<?php
//類的定義以關鍵字class開始，類的命名通常以每個單詞第一個字母大寫
????class?nbaplayer{
????????public?$name?=?"jordan";?//定義屬性
????????public?$height?=?"198cm";
????????public?$team?=?"bull";
????????public?$playernumber?=?"23";
????????//構造函數(shù)，在對象被實例化的時候自動調(diào)用
????????function?__construct($name,$height,$weight,$team){
????????????echo?"it?is?an??nbaplayer?constructor\n";
????????????$this->name?=?$name;
????????????//$this是php里面的偽變量，表示對象自身?？梢酝ㄟ^$this->的方式訪問對象的屬性和方法
????????????$this->height?=?$height;
????????????$this->weight?=?$weight;
????????????$this->team?=?$team;
????????}
????????//定義方法
????public?function?run(){
????????echo?"running\n";
????}
????public?function?dribblr(){
????????echo?"dribbling\n";
????}
????public?function?pass(){
????????echo?"passing\n";
????}
}
????//類到對象的實例化
????//類的實例化為對象時使用關鍵字new，new之后緊跟類的名稱和一對括號
????$jordan?=?new?nbaplayer("jordan","198cm","98kg","bull");???
????//對象中的屬性成員可以通過"->"符號來訪問
????echo?$jordan->name."\n";
????//對象中的成員方法可以通過"->"符號來訪問
????$jordan->dribble();
????$jordan->run();
????//每一次用new實例化對象的時候，都會用類名后面的參數(shù)列表調(diào)用構造函數(shù)
????$james?=?new?nbaplayer("james","203cm","120kg","heat")
????echo?$james->name."\n";
?>

the destructor occurs when all references to an object have been deleted or when the object is explicitly executed when destroyed.

object reference

<?php
//類的定義以關鍵字class開始，類的命名通常以每個單詞第一個字母大寫
????class?NbaPlayer{
????????public?$name?=?"Jordan";?//定義屬性
????????public?$height?=?"198cm";
????????public?$team?=?"Bull";
????????public?$playerNumber?=?"23";

????????//構造函數(shù)，在對象被實例化的時候自動調(diào)用
????????function?__construct($name,$height,$weight,$team){
????????????echo?"It?is?an??NbaPlayer?constructor\n";
????????????$this->name?=?$name;
????????????//$this是PHP里面的偽變量，表示對象自身?？梢酝ㄟ^$this->的方式訪問對象的屬性和方法
????????????$this->height?=?$height;
????????????$this->weight?=?$weight;
????????????$this->team?=?$team;
????????}
????????//析構函數(shù)，在程序執(zhí)行結束的時候會自動調(diào)用
????????//析構函數(shù)通常被用于清理程序使用的資源。比如程序使用了打印機，那么可以再析構函數(shù)里面釋放打印機資源
????????function?__destruct(){
????????????echo?"Destroying".$this->name."\n";
????????}
????????//定義方法
????public?function?run(){
????????echo?"Running\n";
????}
????public?function?dribblr(){
????????echo?"Dribbling\n";
????}
????public?function?pass(){
????????echo?"Passing\n";
????}
}
????//類到對象的實例化
????//類的實例化為對象時使用關鍵字new，new之后緊跟類的名稱和一對括號
????$jordan?=?new?NbaPlayer("Jordan","198cm","98kg","Bull");???
????//對象中的屬性成員可以通過"->"符號來訪問
????echo?$jordan->name."\n";
????//對象中的成員方法可以通過"->"符號來訪問
????$jordan->dribble();
????$jordan->run();
????//每一次用new實例化對象的時候，都會用類名后面的參數(shù)列表調(diào)用構造函數(shù)
????$james?=?new?NbaPlayer("James","203cm","120kg","Heat")
????echo?$james->name."\n";
????//對象的引用用于訪問對象的屬性和方法，$james,$james1和$james2都是對象的引用
????//$james和$james1是對象的兩個獨立引用
????//$james2是$james的影子，使用對象的同一個引用，任何一個賦值為null相當于刪除了同一個引用
????$james1?=?$james;
????$james2?=?&$james
????$james?=?null;
????echo?"from?now?on?James?will?not?be?used.\n"
?>