Pulling the “ID” to the Session when the Button in the Table is Clicked

<?php 

include "config.php";

    if (isset($_POST['update'])) {

        $firstname = $_POST['firstname'];

        $user_id = $_POST['user_id'];

        $lastname = $_POST['lastname'];

        $email = $_POST['email'];

        $password = $_POST['password'];

        $gender = $_POST['gender']; 

        $sql = "UPDATE `users` SET `firstname`='$firstname',`lastname`='$lastname',`email`='$email',`password`='$password',`gender`='$gender' WHERE `id`='$user_id'"; 

        $result = $conn->query($sql); 

        if ($result == TRUE) {

            echo "Record updated successfully.";

        }else{

            echo "Error:" . $sql . "<br>" . $conn->error;

        }

    } 

if (isset($_GET['id'])) {

    $user_id = $_GET['id']; 

    $sql = "SELECT * FROM `users` WHERE `id`='$user_id'";

    $result = $conn->query($sql); 

    if ($result->num_rows > 0) {        

        while ($row = $result->fetch_assoc()) {

            $first_name = $row['firstname'];

            $lastname = $row['lastname'];

            $email = $row['email'];

            $password  = $row['password'];

            $gender = $row['gender'];

            $id = $row['id'];            

        } 

    ?>

        <h2>User Update Form</h2>

        <form action="" method="post">

          <fieldset>

            <legend>Personal information:</legend>

            First name:<br>

            <input type="text" name="firstname" value="<?php echo $first_name; ?>">

            <input type="hidden" name="user_id" value="<?php echo $id; ?>">

            <br>

            Last name:<br>

            <input type="text" name="lastname" value="<?php echo $lastname; ?>">

            <br>

            Email:<br>

            <input type="email" name="email" value="<?php echo $email; ?>">

            <br>

            Password:<br>

            <input type="password" name="password" value="<?php echo $password; ?>">

            <br>

            Gender:<br>

            <input type="radio" name="gender" value="Male" <?php if($gender == 'Male'){ echo "checked";} ?> >Male

            <input type="radio" name="gender" value="Female" <?php if($gender == 'Female'){ echo "checked";} ?>>Female

            <br><br>

            <input type="submit" value="Update" name="update">

          </fieldset>

        </form> 

        </body>

        </html>       

    <?php

    } else{ 

        header('Location: view.php');

    } 

}

?>