php用户注册无法连接数据库

Question

我在让PHP连接到我的数据库时遇到问题。这是我第一次使用PHP，我不确定是什么问题，我在互联网上搜索过而没有找到解决方案。希望你们能帮忙！

这是错误：

  

致命错误：第4行的D：\ wamp64 \ www \ einsteindesigns \ reg.php中找不到“mysqli_connect”类

和代码：

<?php
session_start();
$host = "localhost";
$user = "root";
$pwd = "";
$db = "userdb";

$mysqli = mysqli_connect($host, $user, $pwd, $db);
if(! $mysqli )
{
  die('Could not connect: ' . mysqli_error());
}

if ($_SERVER["REQUEST_METHOD"] == "POST") {

    if ($_POST['password'] == $_POST['confirmpassword']) {

        $first = $_POST['first'];
        $last = $_POST['last'];
        $email = $_POST['email'];
        $password = md5($_POST['password']);
        $confirmpassword = md5($_POST['confirmpassword']);

            $_SESSION['email'] = $email;

               $sql = "INSERT INTO $db (first, last, email, password) "
                      . "VALUES ('$first', '$last', '$email', '$password')";

                    $_SESSION['message'] = "Registration Successful";

                }
                else {
                    $_SESSION['message'] = "Error: User account could not be 
created. Please try again.";
                }
                mysqli_close($mysqli);          
            }
    else {
        $_SESSION['message'] = "Passwords do not match";
    }
?>