Convertir Número a (su forma escrita) Texto

<?php

/**
 * Tomado de:
 *  Copyright 2007-2008 Brenton Fletcher. http://bloople.net/num2text
 *  @link     http://stackoverflow.com/q/277569/367456
 *  @question Is there an easy way to convert a number to a word in PHP?
 *
 * Bajo la licencia de CC-Wiki on Stackoverflow.
 * http://creativecommons.org/licenses/by-sa/3.0/
 *
 *
 * Modificado por kid_goth para uso en regiones de habla española
 * En donde se 1.000.000.000 es igual a Mil millones y no a un
 * billon como se usa en las regiones de habla inglesa. Ademas de agregar
 * POO a la programacion.
 *
 *  @contact Twitter: @_kid_goth
 */
class Num2Text {

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

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

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

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

        return preg_replace('/un mil\b/', 'mil', $output);
    }

    private function convertGroup($index, $valor = '') {
        switch ($index) {
            case 12:
                if ($valor != 'un') {
                    return " sextillones";
                } else {
                    return " sextillon";
                }
            case 11:
                return " mil quintillones";
            case 10:
                if ($valor != 'un') {
                    return " quintillones";
                } else {
                    return " quintillon";
                }
            case 9:
                return " mil cuadrillones";
            case 8:
                if ($valor != 'un') {
                    return " cuadrillones";
                } else {
                    return " cuadrillon";
                }
            case 7:
                return " mil trillones";
            case 6:
                if ($valor != 'un') {
                    return " trillones";
                } else {
                    return " trillon";
                }
            case 5:
                return " mil billones";
            case 4:
                if ($valor != 'un') {
                    return " billones";
                } else {
                    return " billon";
                }
            case 3:
                return " mil millones";
            case 2:
                if ($valor != 'un') {
                    return " millones";
                } else {
                    return " millon";
                }
            case 1:
                return " mil";
            case 0:
                return "";
        }
    }

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

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

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

        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 ($digit1) {
                case "1":
                    return "diez";
                case "2":
                    return "veinte";
                case "3":
                    return "treinta";
                case "4":
                    return "cuarenta";
                case "5":
                    return "cincuenta";
                case "6":
                    return "sesenta";
                case "7":
                    return "setenta";
                case "8":
                    return "ochenta";
                case "9":
                    return "noventa";
            }
        } else if ($digit1 == "1") {
            switch ($digit2) {
                case "1":
                    return "once";
                case "2":
                    return "doce";
                case "3":
                    return "trece";
                case "4":
                    return "catorce";
                case "5":
                    return "quince";
                case "6":
                    return "diez y seis";
                case "7":
                    return "diez y siete";
                case "8":
                    return "diez y ocho";
                case "9":
                    return "diez y nueve";
            }
        } else {
            $temp = $this->convertDigit($digit2);
            switch ($digit1) {
                case "2":
                    return "veinti$temp";
                case "3":
                    return "treinta y $temp";
                case "4":
                    return "cuarenta y $temp";
                case "5":
                    return "cincuenta y $temp";
                case "6":
                    return "sesenta y $temp";
                case "7":
                    return "setenta y $temp";
                case "8":
                    return "ochenta y $temp";
                case "9":
                    return "noventa y $temp";
            }
        }
    }

    private function convertCientos($digit) {
        switch ($digit) {
            case "0":
                return "cero";
            case "1":
                return "ciento";
            case "2":
                return "docientos";
            case "3":
                return "trecientos";
            case "4":
                return "cuatrocientos";
            case "5":
                return "quinientos";
            case "6":
                return "seicientos";
            case "7":
                return "setecientos";
            case "8":
                return "ochocientos";
            case "9":
                return "novecientos";
        }
    }

    private function convertDigit($digit) {
        switch ($digit) {
            case "0":
                return "cero";
            case "1":
                return "un";
            case "2":
                return "dos";
            case "3":
                return "tres";
            case "4":
                return "cuatro";
            case "5":
                return "cinco";
            case "6":
                return "seis";
            case "7":
                return "siete";
            case "8":
                return "ocho";
            case "9":
                return "nueve";
        }
    }

}

/**
 * Ejemplo de Uso:
 *
 * http://localhost/Num2Text.php?n=1000
**/

$objNum2Text = new Num2Text();
$_G = filter_input(INPUT_GET, 'n');
echo $objNum2Text->convertNumber($_G);

// Salida: Mil