$mssqldriver = '{SQL Server Native Client 10.0}';
        $hostname='xxx';
        $dbname='xxx';
        $username='xxx';
        $password='xxx';
        try {
            $dbDB = new \PDO("odbc:Driver=$mssqldriver;Server=$hostname;Database=$dbname", $username, $password);
        } catch (\Exception $e) {
            echo $e->getMessage();
            die;
    }

应该还要安装pdo_odbc，我电脑上还装了SQL Server Native Client

问题已解决，除了以上这些操作以外，还需要对Apache的php.ini把;extension=php_pdo_odbc.dll改为extension=php_pdo_odbc.dll就行