PHP class convert an arbitrary number to an english string

example.php

#! /usr/local/bin/php
<?php

include_once('NumberConv.php');

$a = new NumberConv(0);
echo $a . PHP_EOL;

$a = new NumberConv(5);
echo $a . PHP_EOL;

$a = new NumberConv(10);
echo $a . PHP_EOL;

$a = new NumberConv(13);
echo $a . PHP_EOL;

$a = new NumberConv(16);
echo $a . PHP_EOL;

$a = new NumberConv(20);
echo $a . PHP_EOL;

$a = new NumberConv(305);
echo $a . PHP_EOL;

$a = new NumberConv(500);
echo $a . PHP_EOL;

$a = new NumberConv(1000);
echo $a . PHP_EOL;

$a = new NumberConv(5295200059);
echo $a . PHP_EOL;

$a = new NumberConv(300410);
echo $a . PHP_EOL;

$a = new NumberConv(-1024901490);
echo $a . PHP_EOL;

$a = new NumberConv('$102531');
echo $a . PHP_EOL;

NumberConv.php

<?php

class NumberConv {
    protected $single = [
        1 => 'one',
        2 => 'two',
        3 => 'three',
        4 => 'four',
        5 => 'five',
        6 => 'six',
        7 => 'seven',
        8 => 'eight',
        9 => 'nine',
    ];

    protected $double = [
        10 => 'ten',
        20 => 'twenty',
        30 => 'thirty',
        40 => 'forty',
        50 => 'fifty',
        60 => 'sixty',
        70 => 'seventy',
        80 => 'eighty',
        90 => 'ninety',
    ];

    protected $teen = [
        11 => 'eleven',
        12 => 'twelve',
        13 => 'thirteen',
        14 => 'fourteen',
        15 => 'fifteen',
        16 => 'sixteen',
        17 => 'seventeen',
        18 => 'eighteen',
        19 => 'nineteen',
    ];

    protected $places = [
        1 => 'hundred',
        2 => 'thousand',
        3 => 'million',
        4 => 'billion',
        5 => 'trillion',
        6 => 'quadrillion',
        7 => 'quintillion',
        8 => 'sextillion',
        9 => 'septillion',
        10 => 'octillion',
        11 => 'nonillion',
        12 => 'decillion',
        13 => 'undecillion',
        14 => 'duodecillion',
        15 => 'tredecillion',
    ];

    protected $modifier_map = [
        '-' => [
            'string' => 'negative',
            'fix' => 'prefix'
        ],
        '$' => [
            'string' => 'dollars',
            'fix' => 'suffix'
        ]
    ];

    protected $modifier_value = '';

    public $number = null;

    protected $string = '';


    public function __construct($number) {
        $number = ltrim($number);

        if (strlen($number > 1))
            $number = ltrim($number, '0');

        if (!is_numeric(substr($number, 0, 1))) {
            $this->modifier_value = substr($number, 0, 1);
            $number = substr($number,1);
        }

        $this->number = $number;
    }

    public function __toString() {
        if (!empty($this->string))
            return $this->string;

        if ($this->number == 0) {
            $this->string = 'zero ';
        }
        else {
            /**
             * 1. Reverse the number because numbers actually parse right to left (1234 becomes 4321)
             * 2. Split number into chunks of 3 (4321 becomes [432, 1])
             * 3. Put every element of resulting array back into original order ([432, 1] becomes [234, 1])
             * 4. Reverse the array order ([234, 1] becomes [1, 234])
             */
            $split = array_reverse(array_map('strrev', str_split(strrev($this->number), 3)));
            $places = count($split);

            foreach ($split as $n) {
                if (strlen($n) == 3) {
                    if (array_key_exists($n[0], $this->single))
                        $this->string .= "{$this->single[$n[0]]} {$this->places[1]} ";

                    $n = substr($n,1);
                }

                if (strlen($n) == 2) {
                    if (array_key_exists($n[0] . $n[1], $this->teen)) {
                        $this->string .= $this->teen[$n[0] . $n[1]] . ' ';
                        $n = substr($n,1);
                    }
                    elseif (array_key_exists($n[0] . '0', $this->double)) {
                        $this->string .= $this->double[$n[0] . '0'] . ' ';
                    }

                    $n = substr($n,1);
                }

                if (strlen($n) == 1 && array_key_exists($n[0], $this->single))
                    $this->string .= "{$this->single[$n[0]]} ";

                if ($places > 1 && array_key_exists($places, $this->places))
                    $this->string .= "{$this->places[$places]} ";

                $places--;
            }
        }

        if (!empty($this->modifier_value) && array_key_exists($this->modifier_value, $this->modifier_map)) {
            $modifier = $this->modifier_map["{$this->modifier_value}"];

            if ($modifier['fix'] == 'prefix')
                $this->string = $modifier['string'] . ' ' . $this->string;

            elseif ($modifier['fix'] == 'suffix')
                $this->string .= $modifier['string'];
        }

        return trim($this->string);
    }
}

run_number_conv.php

#! /usr/local/bin/php
<?php

include_once('NumberConv.php');

$a = new NumberConv($argv[1]);
echo $a . PHP_EOL;