PHP putting last inserted row into a session - php

Question

I'm working on a school project, and I'm currently working on a form.
I'm trying to insert the last inserted row from a table in my database into a session, so I can reuse the info when they want to go back to the form to adapt something.
So the data is being inserted into the database, but when I try to put the variable $insertedGegvens into a session, it returns null, and I don't understand why.
I currently have this:
PDO:
public function selectById($id) {
$sql = "SELECT * FROM `gegevens` WHERE `id` = :id";
$stmt = $this->pdo->prepare($sql);
$stmt->bindValue(':id', $id);
$stmt->execute();
}
public function insert($data) {
$errors = $this->validate( $data );
if (empty($errors)) {
$sql = "INSERT INTO `gegevens`
(`naam`, `gemeente`, `postcode`, `adres`,
`huisnr`, `email`, `tel`)
VALUES (:naam, :gemeente, :postcode, :adres,
:huisnr, :email, :tel)";
$stmt = $this->pdo->prepare($sql);
$stmt->bindValue(':naam', $data['naam']);
$stmt->bindValue(':gemeente', $data['gemeente']);
$stmt->bindValue(':postcode', $data['postcode']);
$stmt->bindValue(':adres', $data['adres']);
$stmt->bindValue(':huisnr', $data['huisnr']);
$stmt->bindValue(':email', $data['email']);
$stmt->bindValue(':tel', $data['tel']);
if ($stmt->execute()) {
return $this->selectById($this->pdo->lastInsertId()); //insert returns last row inserted through function selectById
}
}
return false;
}
public function validate( $data ){
$errors = [];
if (!isset($data['naam'])) {
$errors['naam'] = 'Gelieve een naam in te vullen';
}
if (!isset($data['gemeente'])) {
$errors['gemeente'] = 'Gelieve een gemeente in te vullen';
}
if (!isset($data['postcode'])) {
$errors['postcode'] = 'Gelieve een postcode in te vullen';
}
if (empty($data['adres']) ){
$errors['adres'] = 'Gelieve een adres in te vullen';
}
if (empty($data['huisnr']) ){
$errors['huisnr'] = 'Gelieve een huisnummer in te vullen';
}
if (empty($data['email']) ){
$errors['email'] = 'Gelieve een email in te vullen';
}
if (empty($data['tel']) ){
$errors['tel'] = 'Gelieve een telefoonnummer in te vullen';
}
return $errors;
}
if($_POST['action'] == 'toCheckout'){
$insertedGegevens = $this->orderDAO->insert($_POST);
//trying to insert the info it just inserted into a SESSION['user'] so I can reuse this info when user returns to form
$_SESSION['user'] = array();
$_SESSION['user'] = $insertedGegevens;
//$_SESSION['user'] returns null
if (!$insertedGegevens) {
$error = $this->orderDAO->validate($_POST);
$this->set('errors', $error);
}
$_SESSION['user'] = array();
$_SESSION['user'] = $insertedGegevens;
header('Location: index.php?page=checkout');
exit();
}