php Form varibles

Question

我希望有人告诉我这段代码有什么问题。

正在制作表单，但变量未插入用户的sql表

<?php
include 'dbh.php';

$first = $_POST['first'];
$last = $_POST['last'];
$user = $_POST['user'];
$pwd = $_POST['password'];
$sql = "INSERT INTO users(first,last,user,password)
VALUES('$first','$last','$user','$pwd')";
$result=$conn->query($sql);
header("Location: index.php");

?>