这种模式确保一个类只有一个实例并提供一个访问它的全局访问点

Singleton
This class (a) defines an Instance operation that lets clients access its unique instance, and (b) may be responsible for creating its own unique instance.
(这类(a)定义了一个实例操作,允许客户访问其独特的实例,和(b)可能是负责创建自己唯一的实例。)

代码实现

<?php

class Singleton
{
    static private $_instance = null;

    private function __construct() {
    }

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

    /**
    * 防止用户克隆实例
    */
    public function __clone() {
        die('Clone is not allowed.' . E_USER_ERROR);
    }

    public function display() {
        echo "it is a singlton class function";
    }
}

$obj = Singleton::getInstance();
var_dump($obj);
$obj->display();

$obj1 = Singleton::getInstance();
var_dump(($obj === $obj1));