Object Variable Remains Undefined after being Set in Constructor - php

Question

This is the code of my class, only relevant parts of course:
class User {
public $id;
public function __construct($email, $password, $firstName, $lastName) {
$db = Connection::getInstance();
// check if user exists
$id = User::findUserByEmail($email);
if($id > 0){
// echo "User already exists!";
return -1;
}
// Create new row in users table
$stmt = $db->prepare("INSERT INTO `mapdb`.`user` (`email`, `password`, `firstName`, `lastName`)
VALUES (:email, :password, :firstName, :lastName);");
$stmt->bindParam(':email', $email, PDO::PARAM_STR);
$stmt->bindParam(':password', $password, PDO::PARAM_STR);
$stmt->bindParam(':firstName', $firstName, PDO::PARAM_STR);
$stmt->bindParam(':lastName', $firstName, PDO::PARAM_STR);
$stmt->execute();
// check f user added successfully
$newID = User::findUserByEmail($email);
if($newID > 0){
echo "success, ID = ".$newID;
$this->$id = $newID;
// $this->$email = $email;
// $this->$firstName = $firstName;
// $this->$firstName = $firstName;
} else {
echo "failure";
return -1;
}
}
}
And where I actually call the constructor:
$user = new User($email, $password, $firstName, $lastName);
echo "<br>userid: ".$user->id; // (<-- this doesn't echo correctly)
I cannot get the value from the User object whatever I try.
At the moment I get the following error:
Notice: Undefined variable: id
What could possibly deny me access from the variable?

Answer 1

Problem solved, instead of
$this->$id = $newID;
I should have
$this->id = $newID;
Thank goodness for stackoverflow :D