Application.ini:

booking.username = "username"
booking.password = "password"
booking.server   = "http://distribution-xml.booking.com/xml-rpc"
booking.language = "en"

Controller.php:

public function init()
{
    $this->_options = Zend_Registry::get('config')->booking->toArray();
    $this->_booking = new Booking_Service_Booking($this->_options);
}

Usage:

var_dump($this->_booking->getCountries());