PHP Pattern Design Single Example mode

The singleton pattern is designed to prevent multiple instances of duplication, such as duplication of database connections, resulting in wasted resources.

The point is:

1/A class has only one object
2/class is self-created and cannot be declared by new
3/Purpose: This object is uniformly provided throughout the system.

Writing:

1/class defines a private static variable
Only private constructors are available in the 2/class
3/provides a static public function to create the private object itself.
4/Use __clone to prevent cloning

<?php//学习使用单例模式class Danli{    private static $db = null;    private static $num = 0;    private function __construct()    {        echo "对象已经构建完成:".self::$num."次\n";    }    public static function getInstance()    {        if(self::$db instanceof self){            echo "已经实例化,不要重复实例\n";            return self::$db;        }else{            self::$num +=1;            return  self::$db = new self();        }    }    static function test()    {        echo "调用成功\n";    }    public function __clone()    {        echo "不能克隆哦";    }}$d = Danli::getInstance();$d->test();$c = Danli::getInstance();$c->test();$d= clone $c;//

PHP Pattern Design Single Example mode