php PDO实例化一次

class dboperator{	private static $db = null;		public static function getinstance()	{		try 		{			if($db == null)			{				$db = new pdo("mysql:host=".localhost.";dbname=".test,root,root, array(pdo::attr_persistent => true));									$db->setattribute(pdo::attr_case, pdo::case_upper);								}			return $db;		}		catch (exception $e)		{			die("数据库连接失败！".$e->getmessage());		}			}   		function __destruct ()	{		$db = null;	}		/**	 * 次函数主要是以集合的形式返回，或插入集合数据。	 * @param unknown $strsql	 * @param unknown $array	 */    public static function executearraysql($strsql, $array)    {        try         {        	dboperator::getinstance()->begintransaction();        	            $stms = dboperator::getinstance()->prepare($strsql);                        foreach ($array as $value)            {                $stms->execute($value);            }                        dboperator::getinstance()->commit();                    }catch (pdoexception $e)        {            print "error: " . $e->getmessage() . "<br/>";            die();        }            }    /**     * 次函数主要是用来指定条件查询     * @param unknown $strsql     * @param unknown $array 查询参数     */    public static function executesql($strsql,$array)    {        try        {            $stms = dboperator::getinstance()->prepare($strsql);                        $bret = $stms->execute($array);                                    return $bret;                }catch (pdoexception $e)        {            print "error: " . $e->getmessage() . "<br/>";            die();        }    }}