Convert number to its textual name

<?php

/**
 * English Number Converter - Collection of PHP functions to convert a number
 *                            into English text.
 *
 * This exact code is licensed under CC-Wiki on Stackoverflow.
 * http://creativecommons.org/licenses/by-sa/3.0/
 *
 * @link     http://stackoverflow.com/q/277569/367456
 * @question Is there an easy way to convert a number to a word in PHP?
 *
 * This file incorporates work covered by the following copyright and
 * permission notice:
 *
 *   Copyright 2007-2008 Brenton Fletcher. http://bloople.net/num2text
 *   You can use this freely and modify it however you want.
 *
 * Class for POO by kid_goth
 */
class Num2TextEng {

    public function convertNumber($number) {
        $output = "";

        if (preg_match('/\./', $number)) {
            list($integer, $fraction) = explode(".", (string) $number);
        } else {
            $integer = (string) $number;
            $fraction = NULL;
        }

        if ($integer{0} == "-") {
            $output = "negative ";
            $integer = ltrim($integer, "-");
        } else if ($integer{0} == "+") {
            $output = "positive ";
            $integer = ltrim($integer, "+");
        }

        if ($integer{0} == "0") {
            $output .= "zero";
        } else {
            $integer = str_pad($integer, 36, "0", STR_PAD_LEFT);
            $group = rtrim(chunk_split($integer, 3, " "), " ");
            $groups = explode(" ", $group);

            $groups2 = array();
            foreach ($groups as $g) {
                $groups2[] = $this->convertThreeDigit($g{0}, $g{1}, $g{2});
            }

            for ($z = 0; $z < count($groups2); $z++) {
                if ($groups2[$z] != "") {
                    $output .= $groups2[$z] . $this->convertGroup(11 - $z) . (
                            $z < 11 && !array_search('', array_slice($groups2, $z + 1, -1)) && $groups2[11] != '' && $groups[11]{0} == '0' ? " and " : ", "
                            );
                }
            }

            $output = rtrim($output, ", ");
        }

        if ($fraction > 0) {
            $output .= " point";
            for ($i = 0; $i < strlen($fraction); $i++) {
                $output .= " " . $this->convertDigit($fraction{$i});
            }
        }

        return $output;
    }

    private function convertGroup($index) {
        switch ($index) {
            case 11:
                return " decillion";
            case 10:
                return " nonillion";
            case 9:
                return " octillion";
            case 8:
                return " septillion";
            case 7:
                return " sextillion";
            case 6:
                return " quintrillion";
            case 5:
                return " quadrillion";
            case 4:
                return " trillion";
            case 3:
                return " billion";
            case 2:
                return " million";
            case 1:
                return " thousand";
            case 0:
                return "";
        }
    }

    private function convertThreeDigit($digit1, $digit2, $digit3) {
        $buffer = "";

        if ($digit1 == "0" && $digit2 == "0" && $digit3 == "0") {
            return "";
        }

        if ($digit1 != "0") {
            $buffer .= $this->convertDigit($digit1) . " hundred";
            if ($digit2 != "0" || $digit3 != "0") {
                $buffer .= " and ";
            }
        }

        if ($digit2 != "0") {
            $buffer .= $this->convertTwoDigit($digit2, $digit3);
        } else if ($digit3 != "0") {
            $buffer .= $this->convertDigit($digit3);
        }

        return $buffer;
    }

    private function convertTwoDigit($digit1, $digit2) {
        if ($digit2 == "0") {
            switch ($digit1) {
                case "1":
                    return "ten";
                case "2":
                    return "twenty";
                case "3":
                    return "thirty";
                case "4":
                    return "forty";
                case "5":
                    return "fifty";
                case "6":
                    return "sixty";
                case "7":
                    return "seventy";
                case "8":
                    return "eighty";
                case "9":
                    return "ninety";
            }
        } else if ($digit1 == "1") {
            switch ($digit2) {
                case "1":
                    return "eleven";
                case "2":
                    return "twelve";
                case "3":
                    return "thirteen";
                case "4":
                    return "fourteen";
                case "5":
                    return "fifteen";
                case "6":
                    return "sixteen";
                case "7":
                    return "seventeen";
                case "8":
                    return "eighteen";
                case "9":
                    return "nineteen";
            }
        } else {
            $temp = $this->convertDigit($digit2);
            switch ($digit1) {
                case "2":
                    return "twenty-$temp";
                case "3":
                    return "thirty-$temp";
                case "4":
                    return "forty-$temp";
                case "5":
                    return "fifty-$temp";
                case "6":
                    return "sixty-$temp";
                case "7":
                    return "seventy-$temp";
                case "8":
                    return "eighty-$temp";
                case "9":
                    return "ninety-$temp";
            }
        }
    }

    private function convertDigit($digit) {
        switch ($digit) {
            case "0":
                return "zero";
            case "1":
                return "one";
            case "2":
                return "two";
            case "3":
                return "three";
            case "4":
                return "four";
            case "5":
                return "five";
            case "6":
                return "six";
            case "7":
                return "seven";
            case "8":
                return "eight";
            case "9":
                return "nine";
        }
    }

}

/**
 * Example of use:
 *
 * http://localhost/Num2TextEng.php?n=1000
 * */
$objNum2TextEng = new Num2TextEng();
$_G = filter_input(INPUT_GET, 'n');
echo $objNum2TextEng->convertNumber($_G);

// Output: one thousand