获取错误：未定义的索引：

Question

我不知道是否显示此错误

  

注意：第15行的C：\ xampp \ htdocs \ Registration \ server.php中的未定义索引：password_1
  注意：未定义的索引：第16行的C：\ xampp \ htdocs \ Registration \ server.php中的password_2

我的代码是：

session_start();

$username ="";
$email = "";
$errors = array();
//connect to the database
$db = mysqli_connect('localhost' , 'root' ,'' ,'registration');

// if the register button is clicked
if (isset($_POST['register'])) {
    $username = mysqli_real_escape_string($db,$_POST['username']);
    $email = mysqli_real_escape_string($db,$_POST['email']);
    $password_1  = mysqli_real_escape_string($db,$_POST['password_1']);
    $password_2 = mysqli_real_escape_string($db,$_POST['password_2']);



    //ensure that form fields are filled properly
    if (empty($username)) {
        array_push($errors, "Username is required");
    }
    if (empty($email)) {
        array_push($errors, "Email is required");
    }
    if (empty($password_1)) {
        array_push($errors, "Password is required");
    }     
    if ($password_1 != $password_2) {
        array_push($errors, "The two passwords do not match");
    }

    // if there are no errors, save user to database
    if (count($errors) ==0) {
        $password = md5($password_1); 
        $sql = "INSERT INTO users (username, email, password) 
                VALUES ('$username', '$email', '$password')";
        mysqli_query($db, $sql);
        $_SESSION['username'] = $username;
        $_SESSION['success'] = "You are logged in";
        header('location: index.php');
    }

}

你可以帮我解决这个问题吗？