调用未定义的方法mysqli::error(）

我正在尝试制作一个带有图像的教师注册文件。

对我来说，一切似乎都是正确的，但我找不到我犯的错误在哪里。

• 我检查了用户数据是否正确提供
• 我做了一个简单的函数来检查我的图像扩展是否正确
• 我已将所有用户提供的信息分配给变量
• 当我检查时，这些都是正确的
• 我将上传的文件移到了服务器文件夹中，并执行了sql

我的表列似乎是正确的，但每次都会出现错误：

Call to undefined method mysqli::error()

代码：

<?php
require_once("db_const.php");
if (!$_SERVER['REQUEST_METHOD']=='POST' 
    || !$_POST['teacher_submit']=='add_teacher'
    || empty($_POST['teacher_name'])
    || empty($_POST['teacher_username'])
    || empty($_POST['teacher_password'])
    || empty($_POST['teacher_department'])
    || empty($_POST['teacher_phone'])
    || empty($_POST['teacher_email'])
     || empty($_FILES['teacher_image']))
 {
    $_SESSION['error'] = "All fields are required";
    echo "All fields are required";
    header('Location: ../admin.php?page=add-teacher');
    exit;
} else {
   ## connect mysql server
    $mysqli = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);
    # check connection
    if ($mysqli->connect_errno) {
    echo "<p>MySQL error no {$mysqli->connect_errno} :         
   {$mysqli->connect_error}</p>";
    exit();
}

function is_valid_type($file)
{
    // This is an array that holds all the valid image MIME types
    $valid_types = array("image/jpg", "image/jpeg", "image/bmp",   "image/gif", "image/png");
    if (in_array($file['type'], $valid_types))
        return 1;
    return 0;
}

// This variable is the path to the image folder where all the images are going to be stored
// Note that there is a trailing forward slash
$TARGET_PATH = "teacher_photo/";
# prepare data for insertion
$teacher_name               = $_POST['teacher_name'];
$teacher_username           = $_POST['teacher_username'];
$teacher_password           = $_POST['teacher_password'];
$teacher_department         = $_POST['teacher_department'];
$teacher_phone              = $_POST['teacher_phone'];
$teacher_email              = $_POST['teacher_email'];
$teacher_image              = $_FILES['teacher_image'];
$teacher_name           = mysql_real_escape_string($teacher_name);
$teacher_username       = mysql_real_escape_string($teacher_username);
$teacher_password       = mysql_real_escape_string($teacher_password);
$teacher_department     = mysql_real_escape_string($teacher_department);
$teacher_phone          = mysql_real_escape_string($teacher_phone);
$teacher_email          = mysql_real_escape_string($teacher_email);
$teacher_image['name']  =  
 mysql_real_escape_string($teacher_image['name']);

$TARGET_PATH .= $teacher_image['name']; 

 if (!is_valid_type($teacher_image))
 {
    $_SESSION['error'] = "You must upload a jpeg, gif, or bmp";
    echo"You must upload a jpeg, gif, png or bmp";
    exit;
 }

if (file_exists($TARGET_PATH))
{
    $_SESSION['error'] = "A file with that name already exists";
    echo"A file with same name exists already";
    exit;
}
if (move_uploaded_file($teacher_image['tmp_name'], $TARGET_PATH))

{
    // NOTE: This is where a lot of people make mistakes.
    // We are *not* putting the image into the database; we are putting   a reference to the file's location on the server
    $sql = "INSERT INTO teachers (teacher_name, teacher_username,   
 teacher_password, teacher_department, teacher_phone, teacher_email, 
 teacher_image)
    values ('$teacher_name', '$teacher_username', '$teacher_password', 
  '$teacher_department',  '$teacher_phone', '$teacher_email',  '" . 
   "main/teacher_photo/".$teacher_image['name'] . "')";

    $result = $mysqli->query($sql) or die ("Could not insert data into 
 DB: " . $mysqli->error());
     echo "<P>Registration Successfully Completed.";
        echo "<script>setTimeout('"location.href = 
'../admin.php?page=add-teacher';'",3500);</script>";
    exit;
  }
else
  {
// A common cause of file moving failures is because of bad permissions 
 on the directory attempting to be written to
    // Make sure you chmod the directory to be writeable
    $_SESSION['error'] = "Could not upload file. Check read/write 
 persmissions on the directory";
    header("Location: ../admin.php?page=add-teacher");
    exit;
}
}
?>