更新后wamp php 5.6 php插入不起作用

Question

更新后wamp php 5.6 php插入不工作...我做错了什么？ 在更新之前，相同的代码工作正常。

<form action="signup.php" method="POST">
         <input class="insert" name="firstname" required id="firstname-signup" type="text" value="" placeholder="First name"><br><br>
         <input class="insert" name="lastname" required id="lastname-signup" type="text" value="" placeholder="Last neme"><br><br>
         <input class="insert" name="email" required id="email-signup" type="text" value="" placeholder="Email"><br><br>
         <input class="insert" name="password" id="password-signup" type="text" value="" placeholder="Password"><br><br>
 <input class="submit" name="submit" id="button-signup" type="submit" value="Signup">
         </form>


<?php    
     if (isset($_POST['submit'])){

         session_start();
         $firstname = $_POST['firstname'];
         $lastname = $_POST['lastname'];
         $email = $_POST['email'];
         $password = $_POST['password'];

    $sql = "INSERT into user ( firstname, lastname, email, password ) VALUES ('$firstname','$lastname', '$email', '$password' )";
    $result = mysqli_query($con,$sql);

    header('location: login.php');


     }
?>