Detailed explanation of PHP singleton pattern definition and usage examples

This article mainly introduces the definition and use of PHP singleton mode. It analyzes the function, definition, usage and related precautions of PHP singleton mode in detail based on the form of specific database operation classes. Friends in need can refer to it.

The details are as follows:

First, let’s briefly introduce the singleton mode. The singleton mode is to maintain only one instance of a certain class in the application, and it cannot generate a second instance of this class due to the influence of the external environment. Its advantage, from a practical point of view, is that if you maintain the existence of a single data operation class instance in WEB development, you can reduce the consumption of unnecessary redundant connection database resources. For large-scale software development, you can use a singleton to maintain The state of the program synchronizes different operations because the singleton always occupies memory and never has a copy.

For PHP, the most common occasion to use singletons is to write a database operation class. However, when implementing a singleton in PHP, there are the following rules:

1) The singleton class must have a currently declared constructor and be private.

2) The singleton class must have a static variable to store the instance of the class, so that there is only one instance of the singleton class.

3) The singleton class must provide a static method for all other objects to apply this singleton.

Why should the above three conditions be met:

1) Because the singleton class can only be created once when the entire application is running, and whether this creation is done through external calls Complete, but complete itself. Therefore, the singleton class instantiates itself, so its constructor must be private. No other external object can construct a copy of a singleton class again.

2) Because a singleton class can only instantiate itself, and it must provide its own instance for all external applications, there must be one inside the class that can be accessed by the outside world, and it is the only constant access Storage object point, so a static variable must be provided to store the instance object that the singleton class instantiates itself.

3) Because the constructor of the singleton class is private, the singleton class must provide an external interface for the external environment to call the singleton class, so there must be a static method, which can initialize the singleton class or Returns a reference to the object of the singleton class.

A simple example:

class DB{
   private $_link;
   //   保持单例类的静态变量
   static $_instance;
   //   私有的构造函数
   private function __construct(){
       $this->_link = @mysqli_connect(__HOST__, __USER__, __PASSWORD__, __DATABASE__);
       if(! ($this->_link)){
          echo 'Something wrong occurs on the database connection!';  
       }
   }
   //   防止单例类被克隆
   private function __clone(){}
   //   外界访问单例类实例的接口
   public static function getInstance(){
       if(! (self::$_instance instanceof self)){
          self::$_instance = new self();
       }
       return self::$_instance;
   }
}

Note that the __clone() function defined above prevents singleton class objects from being cloned.

The following is also a simple singleton of the database operation class, for reference:

class DB {
   /**
    * the database connection
    * @var   resource
    * @access private
    */
   private $_link;
   /**
    * the static instance of single db
    * @var   object
    * @access static
    */
   static $_instance;
   /**
    * construct the single object
    * @return null
    * @access private
    */
   private function __construct(){
       $this->_link = @mysqli_connect(__HOST__, __USER__, __PASSWORD__, __DATABASE__);
       if(! ($this->_link)){
          echo 'Something wrong occurs on the database connection!';  
       }
   }
   /**
    * empty clone
    * @return null
    * @access private
    */
   private function __clone(){}
   /**
    * for other object to get the instance of db
    * @return self::instance
    * @access public
    */
   public static function getInstance(){
       if(! (self::$_instance instanceof self)){
          self::$_instance = new self();
       }
       return self::$_instance;
   }
   /**
    * query
    * @param  sql string
    * @param  message string
    * @return   resource
    * @access public
    */
   public function query($sql,$message){
       $result = @mysqli_query($this->$_link, $sql) or die($message . mysqli_error($this->$_link));
       return $result;
   }
   /**
    * mysqli_num_rows
    * @param  result resource
    * @return   int
    * @access public
    */
   public function num($result){
       return @mysqli_num_rows($result);
   }
   /**
    * mysqli_fetch_array
    * @param  result resource
    * @return   array
    * @access public
    */
   public function fetchArr($result){
       return @mysqli_fetch_array($result);
   }
   /**
    * mysqli_insert_id
    * @return   int
    * @access public
    */
   public function last_id(){
       return @mysqli_insert_id($this->_link);   
   }
   /**
    * close the database connection
    * @param  result resource
    * @return   null
    * @access public
    */
   public function close(){
       @mysqli_close($this->_link);
   }
   /**
    * fetch once result from the specific sql query
    * @param  sql string
    * @param  message string
    * @return   array
    * @access public
    */
   public function fetchArrOnce($sql, $message){
       $result = $this->query($sql, $message);
       $row = $this->fetchArr($result);
       return $row;
   }
   /**
    * fetch all results from the specific sql query
    * @param  sql string
    * @param  message string
    * @return   array
    * @access public
    */
   public function fetchArrMore($sql, $message){
       $result = $this->query($sql, $message);
       $moreRow = array();
       while($row = $this->fetchArr($result)){
          $moreRow[] = $row;
       }
       return $moreRow;
   }
   /**
    * fetch the number of results from the specific sql query
    * @param  sql string
    * @param  message string
    * @return   array
    * @access public
    */
   public function fetchNum($sql, $message){
       $result = $this->query($sql, $message);
       $resultNum = $this->num($result);
       return $resultNum;
   }
   /**
    * mysqli_prepare
    * @param  sql string
    * @return   stmt object
    * @access public
    */
   public function prepare($sql){
       return @mysqli_prepare($this->_link, $sql);
   }
   /**
    * mysqli_stmt_execute
    * @param  stmt object
    * @param  message string
    * @return   bool
    * @access public
    */
   public function stmt_execute($stmt, $message){
       @mysqli_stmt_execute($stmt) or die($message . mysqli_error($this->_link));
   }
}

Use:

define("__HOST__", "localhost");
define("__USER__", "root");
define("__PASSWORD__", "");
define("__DATABASE__", "eee");
$db = DB::getInstance();

The above is the entire content of this article, I hope it will be helpful to everyone's study.

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