Simple PHP 5.3+ Bcrypt class

Bcrypt.php

<?php

/*
  Originally by Marco Arment <me@marco.org>.
  Edited by Martijn van der Ven <martijn@zegnat.net>.
  This code is released in the public domain.

  THERE IS ABSOLUTELY NO WARRANTY.

  This class is aimed at PHP versions over 5.3 and below 5.5.
  For PHP 5.5 see <http://www.php.net/book.password>.

  Edits:
  1. Removed standalone functions.
  2. Use $2y$ instead of $2a$ when available.
     <http://www.php.net/security/crypt_blowfish.php>
  3. New strict legacy checking (disabled by default),
     when enabled $2a$ hashes will be treated as legacy hashes.
  4. Use a slow string compare to counter timing attacks.
     Always compare 60 characters, the expected length of the hash.

  Todo:
  1. HMAC with a static key before crypt’ing. See:
     <https://wiki.mozilla.org/WebAppSec/Secure_Coding_Guidelines#Password_Storage>
  2. Do not return `crypt` in `hash()` before checking for errors.
*/

class Bcrypt
{
    const DEFAULT_WORK_FACTOR = 8;

    public static function hash($password, $work_factor = 0)
    {
        if (version_compare(PHP_VERSION, '5.3', '<')) throw new Exception('Bcrypt requires PHP 5.3 or above');
        if (!function_exists('openssl_random_pseudo_bytes')) throw new Exception('Bcrypt requires openssl PHP extension');

        if ($work_factor < 4 || $work_factor > 31) $work_factor = self::DEFAULT_WORK_FACTOR;
        $salt =
            '$2' . (version_compare(PHP_VERSION, '5.3.7', '<') ? 'a' : 'y') . '$' .
            str_pad($work_factor, 2, '0', STR_PAD_LEFT) . '$' .
            substr(
                strtr(base64_encode(openssl_random_pseudo_bytes(16)), '+', '.'),
                0, 22
            )
        ;
        return crypt($password, $salt);
    }

    public static function check($password, $stored_hash, $legacy_handler = NULL, $strict = false)
    {
        if (version_compare(PHP_VERSION, '5.3', '<')) throw new Exception('Bcrypt requires PHP 5.3 or above');

        if (self::is_legacy_hash($stored_hash, $strict)) {
            if ($legacy_handler) return call_user_func($legacy_handler, $password, $stored_hash);
            else throw new Exception('Unsupported hash format');
        }

        $password_length = strlen($password = crypt($password, $stored_hash) . chr(0));
        $stored_hash_length = strlen($stored_hash .= chr(0));
        $result = $password_length - $stored_hash_length;
        for ($i = 0; $i < 60; $i++) {
            $result |= (ord($stored_hash[$i % $stored_hash_length]) ^ ord($password[$i % $password_length]));
        }

        return $result === 0;
    }

    public static function is_legacy_hash($hash, $strict = false)
    {
        $prefix = substr($hash, 0, 4);
        return $prefix != '$2y$' && ($prefix != '$2a$') != $strict;
    }
}