PHP: Library Terbilang untuk Bukti Pembayaran atau Kwitansi pada Aplikasi

<?php
/*
 * Copyright 2022 Ahmad Amarullah (https://amarullz.com)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 */

class LibTerbilang
{
    private static function charAt($s, $i)
    {
        return substr($s, $i, 1);
    }
    private static function getValidityNaN($str, $from, $to, $min = 1, $max = 9)
    {
        $val = false;
        $from = ($from < 0) ? 0 : $from;
        for ($i = $from; $i < $to; $i++) {
            if (((int) LibTerbilang::charAt($str, $i) >= $min) && ((int) LibTerbilang::charAt($str, $i) <= $max)) $val = true;
        }
        return $val;
    }
    private static function getTerbilang($i, $str, $len)
    {
        $numA = array("", "satu", "dua", "tiga", "empat", "lima", "enam", "tujuh", "delapan", "sembilan");
        $numB = array("", "se", "dua ", "tiga ", "empat ", "lima ", "enam ", "tujuh ", "delapan ", "sembilan ");
        $numC = array("", "satu ", "dua ", "tiga ", "empat ", "lima ", "enam ", "tujuh ", "delapan ", "sembilan ");
        $numD = array(0 => "puluh", 1 => "belas", 2 => "ratus", 4 => "ribu", 7 => "juta", 10 => "milyar", 13 => "triliun");
        $buf = "";
        $pos = $len - $i;
        switch ($pos) {
            case 1:
                if (!LibTerbilang::getValidityNaN($str, $i - 1, $i, 1, 1))
                    $buf = $numA[(int) LibTerbilang::charAt($str, $i)];
                break;
            case 2:
            case 5:
            case 8:
            case 11:
            case 14:
                if ((int) LibTerbilang::charAt($str, $i) == 1) {
                    if ((int) LibTerbilang::charAt($str, $i + 1) == 0)
                        $buf = ($numB[(int) LibTerbilang::charAt($str, $i)]) . ($numD[0]);
                    else
                        $buf = ($numB[(int) LibTerbilang::charAt($str, $i + 1)]) . ($numD[1]);
                } else if ((int) LibTerbilang::charAt($str, $i) > 1) {
                    $buf = ($numB[(int) LibTerbilang::charAt($str, $i)]) . ($numD[0]);
                }
                break;
            case 3:
            case 6:
            case 9:
            case 12:
            case 15:
                if ((int) LibTerbilang::charAt($str, $i) > 0) {
                    $buf = ($numB[(int) LibTerbilang::charAt($str, $i)]) . ($numD[2]);
                }
                break;
            case 4:
            case 7:
            case 10:
            case 13:
                if (LibTerbilang::getValidityNaN($str, $i - 2, $i)) {
                    if (!LibTerbilang::getValidityNaN($str, $i - 1, $i, 1, 1))
                        $buf = $numC[(int) LibTerbilang::charAt($str, $i)] . ($numD[$pos]);
                    else
                        $buf = $numD[$pos];
                } else if ((int) LibTerbilang::charAt($str, $i) > 0) {
                    if ($pos == 4)
                        $buf = ($numB[(int) LibTerbilang::charAt($str, $i)]) . ($numD[$pos]);
                    else
                        $buf = ($numC[(int) LibTerbilang::charAt($str, $i)]) . ($numD[$pos]);
                }
                break;
        }
        return $buf;
    }

    /**
     * Dapatkan nilai terbilang dari suatu bilangan
     * Contoh:
     * $val = LibTerbilang::terbilang("2000");
     *   menghasilkan: dua ribu
     *
     * @param string $nominal Bilangan yang akan dibaca
     * @return string String Terbilang
     */
    public static function terbilang($nominal)
    {
        $buf = "";
        $str = $nominal . "";
        $len = strlen($str);

        for ($i = 0; $i < $len; $i++) {
            $buf = trim($buf) . " " . (LibTerbilang::getTerbilang($i, $str, $len));
        }
        return trim($buf);
    }
}

<?php
include "libterbilang.php"; /* include Class LibTerbilang */

$a = 5478200;
echo "Terbilang untuk {$a} = " . (LibTerbilang::terbilang($a));

<?php
include "libterbilang.php"; /* include Class LibTerbilang */

/* nilai */
$a = "83956712400";

/* Gunakan ucwords untuk kapital tiap kata */
$kalimat = ucwords(LibTerbilang::terbilang($a));

echo "Anda membayar sebesar: {$kalimat} Rupiah";