Cannot access empty property php class - php

Question

I'm trying to learn OOP, and some of its concept. I've following class for users:
class Users
{
private $host = DB_HOST;
private $user = DB_USERNAME;
private $pass = DB_PASSWORD;
private $dbname = DB_NAME;
private $conn;
private $stmt;
public $error;
function __construct()
{
$dsn = 'mysql:host='.$this->host.';dbname='.$this->dbname.';charset=utf8';
$options = array(
PDO::ATTR_PERSISTENT => true,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
);
try {
$this->conn = new PDO($dsn,$this->user,$this->pass,$options);
} catch (PDOException $e) {
$this->error = $e->getMessage();
}
}
private function mysql_execute_query($sql,$params)
{
$this->stmt = $this->conn->prepare($sql);
$this->stmt->execute($params);
return $this->$stmt;
}
public function find_user_by_provider_uid($provider,$provider_uid)
{
$sql = 'SELECT * FROM users WHERE provider = :provider AND provider_uid = :provider_uid LIMIT 1';
$params = array(
':provider' => $provider,
':provider_uid' => $provider_uid
);
$result = $this->mysql_execute_query($sql,$params);
return $result->fetch();
}
}
First of all is there some tip that comes to mind for structuring this code better? or using more features of oop?
Second, it fails with following error:
PHP Notice: Undefined variable: stmt
PHP Fatal error: Cannot access empty property
Both of this lines refer to return $this->$stmt; inside mysql_execute_query
My hunch is that it has something to do with it being private function. But I cannot tell.
Any ideas?

Answer 1

Here the error:
return $this->$stmt;
But should be:
return $this->stmt;