Why does Call to a member function prepare() on null give an error?

Question

PHP

Good day. I ran into a problem, a very simple code, but for some reason it gives an error and does not return the result of the query, I've been staring at the screen for an hour, but I can't find the error, here is the database connection file and the query method inside it

namespace App;

class Db
{
  protected $dbh;

  public function __construct()
  {
    $config = (include __DIR__.'/../config.php')['db'];
    $options = [
      \PDO::ATTR_ERRMODE  => \PDO::ERRMODE_EXCEPTION,
      \PDO::ATTR_DEFAULT_FETCH_MODE => \PDO::FETCH_ASSOC,
      \PDO::ATTR_EMULATE_PREPARES => false,
    ];
    $dsn = 'mysql:host=' . $config['host'] .'; dbname=' . $config['dbname'];
    $this->$dbh = new \PDO($dsn, $config['login'], $config['password'], $options);
  }

  public function query(string $sql, array $data = [])
  {
    $sth = $this->dbh->prepare($sql);
    $sth->execute($data);
    return $sth->fetchAll();
  }


}

then on the main page I check the performance of such a banal and simple method

require_once __DIR__.'/autoload.php';

$db = new \App\Db();


$data = $db->query('SELECT * FROM about');
var_dump($data);

as a result, I get the error Fatal error: Uncaught Error: Call to a member function prepare() on null in Db.php:22
The connection itself is being created, there are no errors.

Answer 1

$this->$dbh
$this->dbh
find 5 differences
And if the connection to the database does not happen for some other reason, it will be the same.