mysql-PHP,MySQL请问这两行代码有什么区别？

mysqlphp

请问这两行代码有什么区别？我试了一下都可以运行成功。

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

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

完整代码from PHP和MySQL Web开发(原书第4版)：

<code>             <title>TODO supply a title</title>        <meta charset="UTF-8">        <meta name="viewport" content="width=device-width, initial-scale=1.0">                <h1>Book-O-Rama Book Entry Result</h1>        <?php //create short variable names        $isbn=$_POST['isbn'];        $author=$_POST['author'];        $title=$_POST['title'];        $price=$_POST['price'];        if (!$isbn || !$author || !$title || !$price){            echo "You have not entered all the required details. <br />". "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();        ?>    </code>