單列模式保證系統(tǒng)中一個(gè)類只有一個(gè)實(shí)例。即一個(gè)類只有一個(gè)對(duì)象實(shí)例。
特點(diǎn)：
1.一個(gè)類只能有一個(gè)對(duì)象；
2.必須是自行創(chuàng)建這個(gè)類的對(duì)象；
3.要想整個(gè)系統(tǒng)提供這一個(gè)對(duì)象；

<?php 

class single{

    private static $instance;

    public function __construct(){
        echo "自己初始化<br/>";
    }

    //防止克隆對(duì)象
    public function __clone(){}

    /** 實(shí)例化self  */
    public static function getInstance(){
        if(!self::$instance instanceof self){
            self::$instance = new self();
        }
        return self::$instance;
    }

    /* 方法 */
    public function getName(){
        return "name";
    }

}

$obj = single::getInstance();
echo $obj->getName();
?>

單列模式簡(jiǎn)單應(yīng)用：

class mysqlConnect{

    private static $instance;
    private $db;

    public function __construct($config){
        try{
            $this->db = new \PDO($config['host'], $config['user'], $config['password']);
            $this->db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $this->db->exec('set names utf8'); 
        }catch(PDOException $e){
            var_dump($e->getMessage());
        }
    }

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

    public function query(){
        $stmt = $this->db->query('select 1+1 as sum');
        //var_dump($stmt->fetchAll(PDO::FETCH_ASSOC));
        //$res = $stmt->fetchAll(PDO::FETCH_ASSOC);
        $res = $stmt->fetch(PDO::FETCH_ASSOC);
        var_dump($res['sum']);
    }

}

$config = [
    'host' => 'mysql:host=localhost;dbname=demo',
    'user' => 'root',
    'password' => '123456'
];

$obj = mysqlConnect::getInstance($config);
$obj->query();