一个php pdo mysql操作类

<?php
/**
 * PDO 操作
 * Created by PhpStorm.
 * User: sumiaowen
 * Date: 13-11-28
 * Time: 下午9:12
 * To change this template use File | Settings | File Templates.
 */
class Pdo_db
{
 private $dns = null;
 private $username = null;
 private $password = null;
 private $conn = null;
 private static $_instance = null;

 private function __construct($params = array())
 {
  $this->dns      = $params['dns'];
  $this->username = $params['username'];
  $this->password = $params['password'];

  $this->_connect();
 }

 private function __clone() { }

 public function get_instance($params = array())
 {
  if(!(self::$_instance instanceof self))
  {
   self::$_instance = new self($params);
  }

  return self::$_instance;
 }

 private function _connect()
 {
  try
  {
   $this->conn = new PDO($this->dns, $this->username, $this->password);
   $this->conn->query('set names utf8');
  }
  catch(PDOException $e)
  {
   exit('PDOException: ' . $e->getMessage());
  }
 }

 /**
  * 查询一条sql语句
  * @param string $sql
  * @param array  $parameters 需要绑定的参数
  * @param int    $option
  * @return array
  */
 public function query($sql, $parameters = array(), $option = PDO::FETCH_ASSOC)
 {
  $stmt = $this->conn->prepare($sql);
  $stmt->execute($parameters);

  $tmp = array();
  while($row = $stmt->fetch($option))
  {
   $tmp[] = $row;
  }

  return $tmp;
 }

 /**
  * 插入一条数据
  * @param string $sql
  * @param array  $parameters
  * @return int  1 or 0 返回影响行数
  */
 public function insert($sql, $parameters = array())
 {
  $stmt = $this->conn->prepare($sql);
  $stmt->execute($parameters);

  return $stmt->rowCount();
 }

 /**
  * 更新一条数据
  * @param string $sql
  * @param array  $parameters
  * @return int  1 or 0 返回影响行数
  */ www.jb200.com
 public function update($sql, $parameters = array())
 {
  $stmt = $this->conn->prepare($sql);
  $stmt->execute($parameters);

  return $stmt->rowCount();
 }

 /**
  * 删除一条数据
  * @param string $sql
  * @param array  $parameters
  * @return int  1 or 0 返回影响行数
  */
 public function delete($sql, $parameters = array())
 {
  $stmt = $this->conn->prepare($sql);
  $stmt->execute($parameters);

  return $stmt->rowCount();
 }
}