How to update users in the DB by their email

[Burton]

I'm trying to get a users email so it will update them when they purchased premium, I'm using Stripe as my payment system. The only way I can get it to work, is if I insert their email/id into my php page. I'm wanting the page to get the users email on it's on.

$_SESSION['user_id'] = 9;
 
$stripe = array(
    "secret_key" => "sk_test_rDYOBYxCErhrgGCeWQJhR4mQ",
    "publishable_key" => "pk_test_3mIuE3OsajKO4cnFppcDDISu"
    );
 
    \Stripe\Stripe::setApiKey($stripe['secret_key']);
 
$db = new PDO('mysql:host=127.0.0.1;dbname=blog', 'root', '');
 
$userQuery = $db->prepare("
    SELECT id, username, email, premium
    FROM users
    WHERE id = :user_id
    ");
 
    $userQuery->execute(['user_id' => $_SESSION['user_id']]);
 
    $users = $userQuery->fetchObject();
 

I'm using ID's right now, I have to enter them on my own. The website isn't live, just testing. I can get the users email with Stripe on my checkout page by putting

echo $users->email

& the email shows up on the purchase screen, where normally they would have to type it. I'm not sure how to get it on purchase so it will update the user in the DB though.