代码点火器错误未定义索引:

>我有一个来自Codeigniter的错误，它是：

"遇到 PHP 错误

严重性：通知

消息：未定义的索引：域"它不是唯一一个我想使用 id 访问的所有数据都返回相同的错误，但有自己的 id......

我尝试获取表单的值（带有 POST）并将它们放入数据库中。但是，因此我无法访问这些值。我刚刚尝试使用硬编码值，我能够将其记录在数据库中。

在这里，我的视图代码是销售书籍的一种形式：

<form method="post" action="<?php echo 'http://localhost:8888/index.php/sell/addBook/'?>">
        <label for="title">Title</label>
        <input type="text" class="form-control" value="" id="title" placeholder="Title">
        <label for="author">Author</label>
        <input type="text" class="form-control" value="" id="author" placeholder="Author">
        <label for="subject">Subject</label>
        <input type="text" class="form-control" value="" id="subject" placeholder="Subject">
        <label for="price">Price</label>
        <input type="float" class="form-control" value="" id="price" placeholder="Price">
        <label for="code">Course Code</label>
        <input type="text" class="form-control" value="" id="coursecode" placeholder="Course Code">
        <label for="domain">Please select a Domain</label>
        <select id="domain" class="form-control">
            <?php
            $domain = $this->db->get('Domain');
            foreach ($domain->result() as $row){
                echo " <option value='"".$row->name."'">";
                echo $row->name;
                echo "</option>";
            }?>
        </select>
        <label for="coursename">Course Name</label>
        <select id="coursename" class="form-control">
            <?php
            $domain = $this->db->get('Course');
            foreach ($domain->result() as $row){
                echo " <option value='"".$row->name."'">";
                echo $row->name;
                echo "</option>";
            }?>
        </select>
        <label for="university">University</label>
            <select id="university" class="form-control">
                <?php
                $domain = $this->db->get('Course');
                foreach ($domain->result() as $row){
                    echo " <option value='"".$row->college."'">";
                    echo $row->college;
                    echo "</option>";
                }?>
            </select>
        <label for = "selectCondition">Select the Condition of the Book</label> <button type="button" class="btn btn-default btn-xs">
        <span class="glyphicon glyphicon-info-sign" aria-hidden="true"></span>
        </button>
        <select id="condition" class="form-control">
            <option value="mint">Mint</option>
            <option value="very good">Very Good</option>
            <option value="good">Good</option>
            <option value="fair">Fair</option>
            <option value="poor" selected="true">Poor</option>
        </select>
        <br>
        <label for="exampleInputFile">Upload Photo</label>
        <input type="file" id="exampleInputFile">
        <p class="help-block">Upload photos to assure the buyer your book is in good condition</p>
    <button type="submit" class="btn btn-default">Submit</button>
</form>

这里是我的控制器的代码

<?php
class sell extends CI_Controller
{
    public $user = "";
    public function __construct()
    {
        parent::__construct();
        $this->load->helper('url');
        $this->load->database();
    }
    // Store user information and send to profile page
    public function index()
    {
        $this->load->view('sell');
    }
    public function addBook()
    {
        $query1 = "SELECT id FROM Domain WHERE name=" . $_POST['domain']. ";";
        /*$this->db->select('id');
        $this->db->from('Domain');
        $this->db->where('name =', $_POST['domain'));*/
        $domid = $this->db->query($query1);
        $query2 = "SELECT id FROM Course WHERE name=" . $_POST['coursename'] . " AND college=" + $_POST['university'] . ";";
        $course = $this->db->query($query2);

        $this->db->set('title', $_POST['title']);
        $this->db->set('author', $_POST['author']);
        $this->db->set('price', intval($_POST['price']));
        $this->db->set('subject', $_POST['subject']);
        $this->db->set('coursecode', $_POST['coursecode']);
        $this->db->set('cond', $_POST['condition']);
        $this->db->set('domainid', intval($domid));
        $this->db->set('courseid', intval($course));
        $this->db->set('sellerid', $_SESSION['id']);
      
        $this->db->insert('Book');
      
    }
}
?>

谢谢你的帮助！

<select id="domain" class="form-control">

<select id="domain" class="form-control" name="domain">

$query1 = "SELECT id FROM Domain WHERE name='".$_POST['domain']."'";