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 pay FROM payment';
        $result = $this -> $mysqli -> query($sqlQuery);
        while ($row = mysqli_fetch_array($result)){
            echo 'Pay: ' . $row['pay'];
        }
    }
}

require 'DataAccess.php';

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

        $result = $test -> getAll();