class DataAccess{
    private $DB_HOSTNAME = 'local';
    private $USERNAME = 'user';
    private $PASSWORD = 'pass';
    private $DB_NAME = 'db_name';
    private $mysqli;

    public function connect_db() {
        $this -> mysqli = new mysqli($DB_HOSTNAME, $USERNAME, $PASSWORD, $DB_NAME);
        if($this -> mysqli)
            echo "Connection succesful";
        else
            die ("Problem with connection" . mysqli_connect_error());
    }

    public function getAll() {
        $sqlQuery = 'SELECT COUNT(pay) FROM payment';
        $result = $this -> mysqli -> query($sqlQuery);
        return $result;
    }
}

require 'DataAccess.php';

$test = new DataAccess();
$test -> connect_db();

$result = $test -> getAll();
echo $result;

error_reporting(E_ALL);
ini_set('display_errors', 1);