php pdo单例模式怎么同时连接两个数据库？：报错

同一个服务是可以的。贴代码

define("DB_HOST","localhost");
define("DB_NAME","test");
define("DB_USER","root");
define("DB_PASSWORD","root");

Class SafePDO extends PDO {
        public static function exception_handler($exception) {
            die('Uncaught exception: '. $exception->getMessage());
        }
        public function __construct($dsn, $username='', $password='', $driver_options=array()) {
            set_exception_handler(array(__CLASS__, 'exception_handler'));
            parent::__construct($dsn, $username, $password, $driver_options);
            restore_exception_handler();
        }
}

$dsn = 'mysql:host='.DB_HOST.';dbname='.DB_NAME.';charset=utf8';
$opt = array(
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'
    );
$pdo = new SafePDO($dsn,DB_USER,DB_PASSWORD);

$ret = $pdo->query("SELECT * FROM teacher limit 1");

echo "LOCAL DB";

while ($o = $ret->fetch())
{
    print_r($o);
}

echo "OVER DB";

$ret = $pdo->query("SELECT * FROM mysql.user limit 1");

while ($o = $ret->fetch())
{
    print_r($o);
}

可以跨数据库操作。update mysq.user set password='xxx' where user='root';

两个实例, 不可能是一个库啊,  代码发上来.