/**
 * 单例模式
 */
class Db{
    private static $mysqli;//该类中的唯一一个实例
    private function __construct(){}//防止在外部实例化该类
    private function __clone(){}//禁止通过复制的方式实例化该类
    public static function connect(){//数据库连接方法
        self::$mysqli = new MySQLi("localhost","root","","test");
        self::$mysqli->query("set names utf8");
    }
    public static function select($table){//数据库操作方法,后续可以继续完善和添加......
        self::connect();
        $result = self::$mysqli->query("select * from ".$table);
        $result_arr = array();
        while($query = $result->fetch_assoc()){
            $result_arr[]    = $query;
        }
        return $result_arr;//结果集以数组的形式返回
    }
}
$result = Db::select("article");
echo '<pre>';
print_r($result);
echo '</pre>';

class Db{
    private static $_instance = null;//该类中的唯一一个实例
    private $dbConn;
    private function __construct(){//防止在外部实例化该类
        $this->dbConn = new MySQLi("localhost","root","","test");
        $this->dbConn->query("set names utf8");
    }
    private function __clone(){}//禁止通过复制的方式实例化该类
        
    public static function getInstance(){
        if(self::$_instance == null){
            self::$_instance = new self();    
        }
        return self::$_instance;
    }
    
    public function select($table){//数据库操作方法,后续可以继续完善和添加......
        $result = $this->dbConn->query("select * from ".$table);
        $result_arr = array();
        while($query = $result->fetch_assoc()){
            $result_arr[]    = $query;
        }
        return $result_arr;//结果集以数组的形式返回
    }
}
$db = Db::getInstance();
$result = $db->select('article');
echo '<pre>';
print_r($result);
echo '</pre>';