PDO добавление записи в БД. Что делать?

Fatal error:  Uncaught Error: Call to a member function prepare() on null in E:\OSPanel\domains\test.ru\classes\_new.db.php:30
Stack trace:
#0 E:\OSPanel\domains\test.ru\api\ajax.php(19): db->prep('INSERT INTO db_...')
#1 {main}
  thrown in <b>E:\OSPanel\domains\test.ru\classes\_new.db.php</b> on line 30

$db->prep("INSERT INTO db_users_a (user, email, pass, referer, referer_id, date_reg, ip) 
VALUES (?, ?, ?, ?, ?, ?, ?)");
$arr = array('$login', '$email', '$pass', 'admin', '1', '$time', INET_ATON('$ip'));
$db->prep($arr);

<?PHP
class db{
	private $config;
	private $arr;
  public function __construct(Config $config)
  {
  	$this->config = $config;
  	$this->connect();
  }

  private function connect()
  {
  	$host = $this->config->host;
  	$dbname = $this->config->base;
 	$charset = $this->config->charset;
 	$user = $this->config->user;
 	$pass = $this->config->pass;
  	$dsn = "mysql:host=$host;dbname=$dbname;charset=$charset";
  	$opt = [
        PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_LAZY,
        PDO::ATTR_EMULATE_PREPARES   => false,
    ];
  	$pdo = new PDO($dsn, $user, $pass, $opt);
  	return $this;
  }

  public function prep($sql, $arr=array())
  {
  	$stmt = $this->pdo->prepare($sql);
  	return $stmt->execute($arr);
  }
}

?>