Detailed explanation of PHP singleton mode study notes

The singleton mode is a function in PHP that is designed to simplify development and repeated calls. Let me introduce the usage of the singleton mode to you in detail.

1. The concept of singleton pattern

As the name suggests, the singleton pattern has only one instance, and it instantiates itself and provides this instance to the world. What needs to be emphasized is that the singleton pattern

Make sure there can only be one instance of a class!

2. Three key points of singleton pattern

(1) A static variable is needed to save the unique instance of the class

The code is as follows

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代码如下	复制代码
private static $_instance;

private static $_instance;

(2) Constructors and clone functions must be private to prevent users from creating objects and copying instance objects

代码如下	复制代码
private function __construct() { //私有化默认构造方法，防止外界直接实例化 } private function __clone() { //私有化克隆方法，防止用户复制实例 }

The code is as follows	Copy code
private function __construct() { //Private the default construction method to prevent direct instantiation by the outside world } private function __clone() { //Private clone method to prevent users from copying instances }

代码如下	复制代码
public static function getInstance() { if(! (self::$_instance instanceof self) ) { self::$_instance = new self(); } return self::$_instance; }

(3) A public static method (usually getInstance) must be provided to return a reference to a unique instance

3. Reasons for using singleton mode in php
PHP language is an interpreted scripting language. This operating mechanism allows each PHP page to be interpreted and executed, and all related resources

The sources will be recycled. In other words, PHP has no way to make an object resident in memory at the language level, which is different from asp.net, Java, etc.

The

types are different. For example, in Java, singletons will always exist throughout the life cycle of the application. Variables are cross-page level and are truly

To make this instance unique during the application life cycle. However, in PHP, all variables are either global variables or class static

State members are all page-level. Every time the page is executed, a new object will be re-established and will be cleared after the page is executed.

It seems that the PHP singleton mode is meaningless, so I think the PHP singleton mode is only for a single page-level request when multiple

It is very meaningful when the application scenario requires sharing the same object resources.

4. How to implement singleton mode

The code is as follows

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代码如下

复制代码

/**
* 单例模式示例：Demo
*/
class Demo{
//静态成员变量，用来保存全局实例
private static $_instance;
//私有化构造方法，保证外界不能直接实例化
private function __construct(){

}
//私有化克隆方法，防止用户复制实例
private function __clone(){

}
//返还此类的唯一实例
public function getInstance(){
if(!(self::$_instance instanceof self))
{
self::$_instance = new self();
}
return self::$_instance;
}
//这是第一个测试方法
public function test1Function(){
echo '这是第一个测试方法';
}
//这是第二个测试方法
public function test2Function(){
echo '这是第二个测试方法';
}
}
//正确的使用方法
@$demo = Demo::getInstance();
$demo->test1Function();
$demo->test2Function();
//这样实例化会出错，因为构造方法为private
//$demo_new = new Demo;
//复制demo会出错，因为默认的clone方法为private
// $demo_clone = clone $demo;
?>

/**

* Singleton mode example: Demo

*/
class Demo{
//Static member variables, used to save global instances

private static $_instance;

//Private construction method to ensure that the outside world cannot directly instantiate
private function __construct(){

代码如下

复制代码

......
//初始化一个数据库句柄
$db = new DB(...);

//添加用户信息
$db->addUserInfo(...);

......

//在函数中访问数据库，查找用户信息
function getUserInfo()
{
$db = new DB(...);//再次new 数据库类，和数据库建立连接
$db = query(....);//根据查询语句访问数据库
}

} //Private clone method to prevent users from copying instances private function __clone(){ } //Return the only instance of this class public function getInstance(){ if(!(self::$_instance instanceof self)) { Self::$_instance = new self(); } Return self::$_instance; } //This is the first test method public function test1Function(){ echo 'This is the first test method'; } //This is the second test method public function test2Function(){ echo 'This is the second test method'; } } //Correct usage @$demo = Demo::getInstance(); $demo->test1Function(); $demo->test2Function(); //This instantiation will cause an error because the construction method is private //$demo_new = new Demo; //An error will occur when copying the demo because the default clone method is private // $demo_clone = clone $demo; ?> 5. Application occasions of simple interest model (1) Interaction between applications and databases, mostly used for database connections (2) If a class is needed in the system to globally control configuration information, it can be easily implemented using the singleton mode 1. Common database access example:

The code is as follows	Copy code
......<🎜> //Initialize a database handle<🎜> $db = new DB(...);<🎜> <🎜>//Add user information<🎜> $db->addUserInfo(...); ...... //Access the database in the function to find user information function getUserInfo() { $db = new DB(...);//New database class again, establish connection with the database $db = query(....);//Access the database according to the query statement } ?>

2. Apply singleton mode to operate the database:

The code is as follows

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代码如下

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class DB
{
private $_db;
private static $_instance;

private function __construct(...)
{
$this->_db = pg_connect(...);//postgrsql
    }

    private function __clone() {}; //覆盖__clone()方法，禁止克隆

    public static function getInstance()
    {
        if(! (self::$_instance instanceof self) ) {
            self::$_instance = new self();
        }
        return self::$_instance;
    }

public function addUserInfo(...)
{

}

public function getUserInfo(...)
{

}

//test

$db = DB::getInstance();

$db->addUserInfo(...);

$db->getUserInfo(...);

<🎜>class DB
{
Private $_db;
Private static $_instance;

Private function __construct(...)
{
            $this->_db = pg_connect(...);//postgrsql
}

Private function __clone() {}; //Override the __clone() method and prohibit cloning

Public static function getInstance()
{
If(! (self::$_instance instanceof self) ) {
              self::$_instance = new self();
                                                              return self::$_instance;
}

public function addUserInfo(...)
{ } public function getUserInfo(...)
{ } } //test $db = DB::getInstance(); $db->addUserInfo(...); $db->getUserInfo(...);
?>

In-depth understanding

代码如下

复制代码

class db {
public $conn;
public static $sql;
public static $instance=null;
private function __construct(){
  require_once('db.config.php');
  $this->conn = mysql_connect($db['host'],$db['user'],$db['password']);
  if(!mysql_select_db($db['database'],$this->conn)){
   echo "失败";
  };
  mysql_query('set names utf8',$this->conn);
}
public static function getInstance(){
  if(is_null(self::$instance)){
   self::$instance = new db;
  }
  return self::$instance;
}
/**
* Query database
*/
public function select($table,$condition=array(),$field = array()){
  $where='';
  if(!empty($condition)){

   foreach($condition as $k=>$v){
    $where.=$k."='".$v."' and ";
   }
   $where='where '.$where .'1=1';
  }
  $fieldstr = '';
  if(!empty($field)){

   foreach($field as $k=>$v){
    $fieldstr.= $v.',';
   }
    $fieldstr = rtrim($fieldstr,',');
  }else{
   $fieldstr = '*';
  }
  self::$sql = "select {$fieldstr} from {$table} {$where}";
  $result=mysql_query(self::$sql,$this->conn);
  $resuleRow = array();
  $i = 0;
  while($row=mysql_fetch_assoc($result)){
   foreach($row as $k=>$v){
    $resuleRow[$i][$k] = $v;
   }
   $i++;
  }
  return $resuleRow;
}
/**
* Add a record
*/
public function insert($table,$data){
$values = '';
$datas = '';
foreach($data as $k=>$v){
  $values.=$k.',';
  $datas.="'$v'".',';
}
$values = rtrim($values,',');
$datas   = rtrim($datas,',');
self::$sql = "INSERT INTO {$table} ({$values}) VALUES ({$datas})";
  if(mysql_query(self::$sql)){
   return mysql_insert_id();
  }else{
   return false;
  };
}
/**
* Modify a record
*/
public function update($table,$data,$condition=array()){
  $where='';
  if(!empty($condition)){

   foreach($condition as $k=>$v){
    $where.=$k."='".$v."' and ";
   }
   $where='where '.$where .'1=1';
  }
  $updatastr = '';
  if(!empty($data)){
   foreach($data as $k=>$v){
    $updatastr.= $k."='".$v."',";
   }
   $updatastr = 'set '.rtrim($updatastr,',');
  }
  self::$sql = "update {$table} {$updatastr} {$where}";
  return mysql_query(self::$sql);
}
/**
* Delete record
*/
public function delete($table,$condition){
$where='';
  if(!empty($condition)){

   foreach($condition as $k=>$v){
    $where.=$k."='".$v."' and ";
   }
   $where='where '.$where .'1=1';
  }
  self::$sql = "delete from {$table} {$where}";
  return mysql_query(self::$sql);

}

public static function getLastSql(){
echo self::$sql;
}

}

$db = db::getInstance();
//$list = $db->select('demo',array('name'=>'tom','password'=>'ds'),array

('name','password'));
//echo $db->insert('demo',array('name'=>'You've been there lately','password'=>'123'));
//echo $db->update('demo',array("name"=>'xxx',"password"=>'123'),array('id'=>1));
echo $db->delete('demo',array('id'=>'2'));
db::getLastSql();
echo "

";

?>

There are many design patterns in php, among which the singleton pattern is a more commonly used pattern when we write code. It can not only

It is effective enough to reduce the resource consumption of new operations, and it can easily control certain global configuration information! I hope everyone can learn PHP

Xi Zhong has a deep understanding of the application of the singleton pattern.

Related labels：

php for Function Singleton study develop yes model use notes simplify Detailed explanation

source：php.cn

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Detailed explanation of PHP singleton mode study notes_PHP tutorial