PHP验证脚本

Question

我已经设置了一个系统，用户可以通过该系统收到他们点击按钮的电子邮件，它将验证他们的电子邮件地址。它将以下url变量传递给页面：

verify.php?email=testemail@gmail.com&first_name=Test&last_name=User

当页面加载时，信息将传递到数据库并存储，但之后会出现错误消息？我做错了什么？

PHP脚本（verify.php）

<?php
include("admin/dbh.php");

$email = $_GET['email'];
$first_name = $_GET['first_name'];
$last_name = $_GET['last_name'];
$today = date("Y/m/d");
$sql = "SELECT * FROM subscribersList WHERE email=$email LIMIT 1";

$result = $conn->query($sql);
if($result -> num_rows){
    echo "<h1>This email address is already signed up to our mailing list!</h1>";
}else{
    $sql = "INSERT INTO subscribersList (first_name, last_name, email, verified, subDate) VALUES ('$first_name', '$last_name', '$email', 1, '$today')";
    $result = $conn -> query($sql);
    header("location: index.php");
}
?>