<?php
/*
 * 单例数据库连接
 */
class Db {
  private static $_instance; //static可以保存值不丢失
  private static $_dbConnect;
  private $_dbConfig = array(
    'host' => '127.0.0.1',
    'user' => 'root',
    'password' => '',
    'database' => 'yii2basic',
  );//保存数据库的配置信息

  //使用private防止用户new
  private function __construct(){

  }

  //重写clone防止用户进行clone
  public function __clone(){
    //当用户clone操作时产生一个错误信息
    trigger_error("Can't clone object",E_USER_ERROR);
  }  

  //由类的自身来进行实例化 (作者：码农教程 http://www.manongjc.com )
  public static function getInstance(){
    if(!(self::$_instance instanceof self)){
      self::$_instance = new self();
    }
    return self::$_instance;
  }

  public function connect(){
    self::$_dbConnect = @mysql_connect($this->_dbConfig['host'],
      $this->_dbConfig['user'],$this->_dbConfig['password']);

    if(!self::$_dbConnect){
      throw new Exception("mysql connect error".mysql_error());
      //die("mysql connect error".mysql_error());
    }

    mysql_query("SET NAMES UTF8");
    mysql_select_db($this->_dbConfig['database'],self::$_dbConnect);
    return self::$_dbConnect;
  }
}

$a = Db::getInstance();
try{
  $a->connect();
}catch(Exception $e){
  echo "sorry,error was happend.".$e->getMessage();
}