A codeigniter library to convert number into words

Numbertowords.php

<?php
if (!defined('BASEPATH'))
  exit('No direct script access allowed');

class Numbertowords {

	function convert_number($number) {
		if (($number < 0) || ($number > 999999999)) {
			throw new Exception("Number is out of range");
		}

		$Gn = floor($number / 1000000);
		/* Millions (giga) */
		$number -= $Gn * 1000000;
		$kn = floor($number / 1000);
		/* Thousands (kilo) */
		$number -= $kn * 1000;
		$Hn = floor($number / 100);
		/* Hundreds (hecto) */
		$number -= $Hn * 100;
		$Dn = floor($number / 10);
		/* Tens (deca) */
		$n = $number % 10;
		/* Ones */

		$res = "";

		if ($Gn) {
			$res .= $this->convert_number($Gn) .  "Million";
		}

		if ($kn) {
			$res .= (empty($res) ? "" : " ") .$this->convert_number($kn) . " Thousand";
		}

		if ($Hn) {
			$res .= (empty($res) ? "" : " ") .$this->convert_number($Hn) . " Hundred";
		}

		$ones = array("", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eightteen", "Nineteen");
		$tens = array("", "", "Twenty", "Thirty", "Fourty", "Fifty", "Sixty", "Seventy", "Eigthy", "Ninety");

		if ($Dn || $n) {
			if (!empty($res)) {
				$res .= " and ";
			}

			if ($Dn < 2) {
				$res .= $ones[$Dn * 10 + $n];
			} else {
				$res .= $tens[$Dn];

				if ($n) {
					$res .= "-" . $ones[$n];
				}
			}
		}

		if (empty($res)) {
			$res = "zero";
		}

		return $res;
	}

}
?>

In above comment I improved opnchaudhary's code to read points. Now it will give currency as "Cents" at the end.

Hello. Code is very nice and working properly. how can i have a "billion" integrated in the code