Math Captcha class

MathCaptcha.php

<?php
/**
 * Math Captcha class.
 *
 * Generates a simple, plain text math equation as an alternative to image-based CAPTCHAs.
 *
 * require __DIR__.'/../mathcaptcha.php';
 *
 * $captcha = new MathCaptcha();
 *
 * $captcha = new MathCaptcha(array('+', '-'), 1, 5, 3);
 *
 */
class MathCaptcha {

    public $variables = array();
    public $equation = null;
    public $result  = null;
    public $result_hash  = null;

    private $operands = array();
    private $minNumber;
    private $maxNumber;
    private $numberOfVariables;

    public function __construct($operands = array('+', '-', '*'), $minNumber = 1, $maxNumber = 9, $numberOfVariables = 2) {
        $this->operands = $operands;
        $this->minNumber = $minNumber;
        $this->maxNumber = $maxNumber;
        $this->numberOfVariables = $numberOfVariables;

        $this->generateEquation();

        $this->result = $this->generateAnswer();
        $this->result_hash = self::hash($this->result);
    }

    private function generateEquation() {
        //shuffle($this->operands);
        for($i = 0; $i < $this->numberOfVariables; $i++) {
            if ($i > 0) {
               $this->variables[] = $this->operands[array_rand($this->operands, 1)];
            }
            $this->variables[] = rand($this->minNumber, $this->maxNumber);
        }

        $this->equation = implode(' ', $this->variables);
    }

    private function generateAnswer() {
        $a = create_function("", "return (".$this->equation.");" );
        return $a();
    }

    static function hash($result) {
        //return md5($result);
        return crypt($result);

    }

    static function check($result, $hash) {
        //return md5($result) == $hash;
        return crypt($result, $hash) == $hash;
    }
}
?>