请问这两行代码有什么区别？我试了一下都可以运行成功。

 $query = "insert into books values (' ".$isbn." ', ' ".$author." ', ' ".$title." ', ' ".$price." ')";

$query = "insert into books values (' $isbn ', ' $author ', ' $title', ' $price ')";` 

完整代码from PHP和MySQL Web开发(原书第4版)：

                                             
Book-O-Rama Book Entry Result
        ". "Please go back and try again";            exit;        }        if (!get_magic_quotes_gpc()){            $isbn = addslashes($isbn);            $author = addslashes($author);            $title = addslashes($title);            $price = doubleval($price);        }        @ $db = new mysqli('localhost', 'root', '123', 'mydb');        if (mysqli_connect_errno()){            echo "Error: Could not connect to database. Please try again later.";            exit;        }        $query = "insert into books values (' ".$isbn." ', ' ".$author." ', ' ".$title." ', ' ".$price." ')";        $result = $db->query($query);        if ($result) {            echo $db->affected_rows. "book inserted into database.";        }  else {                echo "An error has occurred. The item was not added.";        }        $db->close();        ?>