PHP量级数据遍历方案：yield生成器

<?php
/**
 * Created by PhpStorm.
 * User: zrj
 * Date: 18-6-22
 * Time: 上午10:49
 */
declare(strict_types=1);//开启强类型模式

class DB
{
    private static $instance = null;
    private static $sth = null;

    private function __construct()
    {
        try {
            $dsn = "mysql:host=192.168.1.9;dbname=yundou_v3";
            static::$instance = new \PDO($dsn, 'user', 'password');
        } catch (PDOException $e) {
            echo 'Connection failed: ' . $e->getMessage();
            exit();
        }
        static::$instance->setAttribute(\PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);
    }

    public static function getInstance()
    {
        if (!(static::$instance instanceof \PDO)) {
            new static();
        }
        return static::$instance;
    }

    public static function excuteQuery(string $sql)
    {
        static::$sth = static::$instance->prepare($sql);
        static::$sth->execute();
    }

    //防止实例被克隆
    private function __clone()
    {
    }

    public static function cursor()
    {
        while ($row = static::$sth->fetch(\PDO::FETCH_ASSOC)) {
            yield $row;
        }
    }
}

DB::getInstance();
DB::excuteQuery("select * from bill_info");
foreach (DB::cursor() as $info) {
    echo $info['id'] . PHP_EOL;
}