The concept and characteristics of PHP singleton mode

This article mainly introduces the concept and characteristics of PHP singleton mode. Interested friends can refer to it. I hope it will be helpful to everyone.

The concept of singleton mode

The singleton mode refers to a design pattern in which a certain class has only one object instance in the entire application. Specifically, as a method of object creation, the singleton pattern ensures that a class has only one instance, instantiates itself and provides this instance globally to the entire system. It does not create a copy of the instance, but returns a reference to the instance stored internally in the singleton class.

Characteristics of singleton mode

The main feature of singleton mode is "Three private and one public":
Requires a saving class The private static member variable of the only instance
The constructor must be declared as private to prevent an external program from newing an object and thereby losing the meaning of a singleton
The clone function must be declared Being private, preventing the object from being cloned
must provide a public static method (usually named getInstance) to access this instance, thereby returning a reference to the unique instance.

Reasons and scenarios for using the singleton mode

In most applications of PHP, there will be a large number of database operations. If the singleton mode is not used, then every time New operation is required, but each new will consume a lot of system resources and memory resources, and each time the database is opened and closed, it is a great test and waste of the database. Therefore, the singleton pattern is often used in database operation classes.
Similarly, if a class is needed to globally control certain configuration information in the system, it can be easily implemented using the singleton mode.

PHP singleton mode implementation

The following is a framework for PHP singleton mode to implement database operation classes

<?php
 class Db{
 const DB_HOST=&#39;localhost&#39;;
 const DB_NAME=&#39;&#39;;
 const DB_USER=&#39;&#39;;
 const DB_PWD=&#39;&#39;;
 private $_db;
 //保存实例的私有静态变量
 private static $_instance;
 //构造函数和克隆函数都声明为私有的
 private function __construct(){
  //$this->_db=mysql_connect();
 }
 private function __clone(){
  //实现
 }
 //访问实例的公共静态方法
 public static function getInstance(){
  if(!(self::$_instance instanceof self)){
  self::$_instance=new self();
  }
  //或者
  if(self::$_instance===null){
  self::$_instance=new Db();
  }
  return self::$_instance;
 }
 public function fetchAll(){
  //实现
 }
 public function fetchRow(){
  //实现
 }
 }
 //类外部获取实例的引用
 $db=Db::getInstance();
?>

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