Convert any positive integer to English

numberToWords.php

<?php
    $_1to19 = [
        "one",
        "two",
        "three",
        "four",
        "five",
        "six",
        "seven",
        "eight",
        "nine",
        "ten",
        "eleven",
        "twelve",
        "thirteen",
        "fourteen",
        "fifteen",
        "sixteen",
        "seventeen",
        "eighteen",
        "nineteen",
    ];
    $_teen = [
        "twenty",
        "thirty",
        "forty",
        "fifty",
        "sixty",
        "seventy",
        "eighty",
        "ninety",
    ];
    $_mult = [
        2  => 'hundred',
        3  => 'thousand',
        6  => 'million',
        9  => 'billion',
        12 => 'trillion',
        15 => 'quadrillion',
        18 => 'quintillion',
        21 => 'sextillion',
        24 => 'septillion', // php can't count this high
        27 => 'octillion',
    ];
    $fnBase = function ($n, $x) use (&$fn, $_mult) {
        return $fn($n / (10 ** $x)) . ' ' . $_mult[$x];
    };
    $fnOne = function ($n, $x) use (&$fn, &$fnBase) {
            $y = ($n % (10 ** $x)) % (10 ** $x);
            $s = $fn($y);
            $sep = ($x === 2 && $s ? " and " : ($y < 100 ? ($y ? " and " : '') : ', '));
            return $fnBase($n, $x) . $sep . $s;
        };
        $fnHundred = function ($n, $x) use (&$fn, &$fnBase) {
            $y = $n % (10 ** $x);
            $sep = ($y < 100 ? ($y ? ' and ' : '') : ', ');
            return ', ' . $fnBase($n, $x) . $sep . $fn($y);
        };
        $fn = function ($n) use (&$fn, $_1to19, $_teen, $number, &$fnOne, &$fnHundred) {
            switch ($n) {
                case 0:
                    return ($number > 1 ? '' : 'zero');
                case $n < 20:
                    return $_1to19[$n - 1];
                case $n < 100:
                    return $_teen[($n / 10) - 2] . ' ' . $fn($n % 10);
                case $n < (10 ** 3):
                    return $fnOne($n, 2);
            };
            for ($i = 4; $i < 27; ++$i) {
                if ($n < (10 ** $i)) {
                    break;
                }
            }
            return ($i % 3) ? $fnHundred($n, $i - ($i % 3)) : $fnOne($n, $i - 3);
        };
        $number = $fn((int)$number);
        $number = str_replace(', , ', ', ', $number);
        $number = str_replace(',  ', ', ', $number);
        $number = str_replace('  ', ' ', $number);
        $number = ltrim($number, ', ');

        return $number;
    $fn = function ($n) use (&$fn, $_1to19, $_teen, $number, &$fnOne, &$fnHundred) {
        switch ($n) {
            case 0:
                return ($number > 1 ? '' : 'zero');
            case $n < 20:
                return $_1to19[$n - 1];
            case $n < 100:
                return $_teen[($n / 10) - 2] . ' ' . $fn($n % 10);
            case $n < (10 ** 3):
                return $fnOne($n, 2);
        };
        for ($i = 4; $i < 27; ++$i) {
            if ($n < (10 ** $i)) {
                break;
            }
        }
        return ($i % 3) ? $fnHundred($n, $i - ($i % 3)) : $fnOne($n, $i - 3);
    };
    $number = $fn((int)$number);

    return $number;