PHP實現(xiàn)單例模式建立數(shù)據(jù)庫連接的方法分析

/**
 * 單例模式連接數(shù)據(jù)庫--面向?qū)ο?
 * */
//final關(guān)鍵字阻止此類被繼承
final class sql2
{
  static $instance;
  static $connect;
  protected $result;
  //protected關(guān)鍵字阻止此類在外部進行實例化
  protected function __construct($host, $user, $password)
  {
    self::$connect = @new mysqli($host, $user, $password);
    if (self::$connect->connect_errno) {
      die(iconv('gbk', 'utf-8', self::$connect->connect_error) . '(' . self::$connect->connect_errno . ')');
    }
  }
  //protected關(guān)鍵字阻止此類在外部進行克隆
  protected function __clone()
  {
  }
  //當對象被銷毀時關(guān)閉連接
  function __destruct()
  {
    self::$connect->close();
  }
  //獲取實例
  static function getInstance($host, $user, $password)
  {
    self::$instance = self::$instance ?: new self($host, $user, $password);
    return self::$instance;
  }
  //選擇數(shù)據(jù)庫
  function set_db($db)
  {
    if (!self::$connect->select_db($db)) {
      die(iconv('gbk', 'utf-8', self::$connect->error) . '(' . self::$connect->errno . ')');
    }
  }
  //執(zhí)行SQL語句
  function query($query)
  {
    if (!($re = self::$connect->query($query))) {
      die(iconv('gbk', 'utf-8', self::$connect->error) . '(' . self::$connect->errno . ')');
    }
    $this->result = $re;
    return $re;
  }
  //以數(shù)組形式返回查詢結(jié)果
  function fetch_arr($query)
  {
    $re = $this->query($query);
    $res = [];
    while ($row = $re->fetch_assoc()) {
      $res[] = $row;
    }
    return $res;
  }
  //獲取記錄數(shù)
  function get_row()
  {
    return $this->result->num_rows;
  }
}
$ins = sql2::getInstance('127.0.0.1', 'root', 'root');
$ins->set_db('houtai');
$re = $ins->fetch_arr('select * from user ');
//var_dump($re);
$ins->get_row();