PHP Bcrypt class

bcrypt.php

<?php
/**
 * From http://stackoverflow.com/a/6337021
 */
class Bcrypt {
    private $rounds;

    public function __construct($rounds = 12) {
        if (CRYPT_BLOWFISH != 1) {
            throw new Exception("bcrypt not supported in this installation. See http://php.net/crypt");
        }
        $this->rounds = $rounds;
    }

    public function hash($input) {
        $hash = crypt($input, $this->getSalt());
        if (strlen($hash) > 13)
            return $hash;

        return false;
    }

    public function verify($input, $existingHash) {
        $hash = crypt($input, $existingHash);
        return $hash === $existingHash;
    }

    private function getSalt() {
        $salt = sprintf('$2a$%02d$', $this->rounds);
        $bytes = $this->getRandomBytes(16);
        $salt .= $this->encodeBytes($bytes);
        return $salt;
    }

    private $randomState;

    private function getRandomBytes($count) {
        $bytes = '';

        if (function_exists('openssl_random_pseudo_bytes') &&
                (strtoupper(substr(PHP_OS, 0, 3)) !== 'WIN')
        ) { // OpenSSL slow on Win
            $bytes = openssl_random_pseudo_bytes($count);
        }

        if ($bytes === '' && is_readable('/dev/urandom') &&
                ($hRand = @fopen('/dev/urandom', 'rb')) !== FALSE
        ) {
            $bytes = fread($hRand, $count);
            fclose($hRand);
        }

        if (strlen($bytes) < $count) {
            $bytes = '';

            if ($this->randomState === null) {
                $this->randomState = microtime();
                if (function_exists('getmypid')) {
                    $this->randomState .= getmypid();
                }
            }

            for ($i = 0; $i < $count; $i += 16) {
                $this->randomState = md5(microtime() . $this->randomState);

                if (PHP_VERSION >= '5') {
                    $bytes .= md5($this->randomState, true);
                } else {
                    $bytes .= pack('H*', md5($this->randomState));
                }
            }

            $bytes = substr($bytes, 0, $count);
        }

        return $bytes;
    }

    private function encodeBytes($input) {
        // The following is code from the PHP Password Hashing Framework
        $itoa64 = './ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

        $output = '';
        $i = 0;
        do {
            $c1 = ord($input[$i++]);
            $output .= $itoa64[$c1 >> 2];
            $c1 = ($c1 & 0x03) << 4;
            if ($i >= 16) {
                $output .= $itoa64[$c1];
                break;
            }

            $c2 = ord($input[$i++]);
            $c1 |= $c2 >> 4;
            $output .= $itoa64[$c1];
            $c1 = ($c2 & 0x0f) << 2;

            $c2 = ord($input[$i++]);
            $c1 |= $c2 >> 6;
            $output .= $itoa64[$c1];
            $output .= $itoa64[$c2 & 0x3f];
        } while (1);

        return $output;
    }
}