Class for getting the spelled out string of a number (was a coding challenge once)

NumberSpell.class.php

<?php

class NumberSpell
{
    public $number;
    public $spelled;

    private $rules = array(
        0 => array('one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine'),
        1 => array(null, 'twen', 'thir', 'for', 'fif', null, null, null, null)
    );
    private $exceptions = array(
        10 => 'ten',
        11 => 'eleven',
        12 => 'twelve',
        14 => 'fourteen'
    );

    public function __construct($number) {
        if (is_float($number) || (!is_int($number) && !is_numeric($number))) {
            throw new InvalidArgumentException('Only integers or strings with an integer supported.');
        }

        $this->number = (int) $number;
        $spellData = $this->transformToSpell($number);
        $this->spelled = implode(' ', array_reverse(array_filter($spellData)));
    }

    public function transformToSpell($number) {
        if (isset($this->exceptions[$number])) {
            return array($this->exceptions[$number]);
        }

        $digits = str_split(strrev((string) $number));
        $spelledDigits = array();

        foreach ($digits as $pos => $digit) {
            if ($pos == 0) {
                if (!isset($digits[1]) || $digits[1] != 1) {
                    $spelledDigits[0] = ($digit > 0) ? $this->getSpelling($digit, $pos) : '';
                }
            } elseif ($pos == 1) {
                if (isset($this->exceptions[$digit . $digits[0]])) {
                    $spelledDigits[1] = $this->exceptions[$digit . $digits[0]];
                    continue;
                }

                $digitContext = ($digit == 1) ? $digits[0] : $digit;

                if ($digit > 0) {
                    $spelledDigits[1] = rtrim($this->getSpelling($digitContext, $pos), 't');
                    $spelledDigits[1] .= ($digit == 1) ? 'teen' : 'ty';
                }
            } elseif ($pos > 1 && $digit > 0) {
                $spelledDigits[$pos] = $this->getSpelling($digit, 0);
                $spelledDigits[$pos] .= ($pos == 2) ? ' hundred' : ' thousand';
            }
        }

        return $spelledDigits;
    }

    private function getSpelling($value, $pos) {
        if ($value > 9 || $value < 1) {
            return false;
        }

        $value--;

        if (isset($this->rules[$pos][$value]) && $this->rules[$pos][$value] != null) {
            return $this->rules[$pos][$value];
        } else {
            return $this->rules[0][$value];
        }
    }

    public function __toString() {
        return $this->spelled;
    }
}