用php写单列模式

用单列模式写一个链接数据库的例子

<?php
// http://php.net/manual/zh/book.pdo.php

class DB {

    private static $dbInstance;

    private $dbCon;

    private function __construct() {

        try {
            $dbConf = self::getConf();
            $this->dbCon = new PDO($dbConf['dsn'], $dbConf['username'], $dbConf['password']);
        } catch (PDOException $e) {
            die('Could not connect: ' . $e->getMessage());
        }

    }

    public static function getInstance() {

        if (!(self::$dbInstance instanceof self)) {
            self::$dbInstance = new self();
        }
        return self::$dbInstance;
    }

    public function __clone() {
        throw new Exception("Can't clone");
    }

    public function getDbCon()  
    {  
        return $this->dbCon;
    }

    // return arr
    public function select() {
        $stmt = $this->getDbCon()->prepare("select * from users limit 1");
        $stmt->execute();
        return $stmt->fetchAll();
    }

    public static function getConf()
    {
        return $dbConf = [
            'dsn' => 'mysql:dbname=blog;host=127.0.0.1',
            'username' => 'root',
            'password' => 'password'
        ];
    }

}

$db = DB::getInstance();
$rows = $db->select();
var_dump($rows);

在建立数据库连接之前检查数据库连接是否已经存在，也就是说同一个客户端请求连接数据库只实例化一次，避免了多次链接数据库导致内存增长。

单列模式必须满足， private __construct, private instance, can't clone, public getInstance. 只能通过getInstance方法获取实例。

使用单列模式类似的场景有：
记录log的类，战争游戏项目中某个大类的英雄，某个大类的API初始化。