PHP Bcrypt class

Bcrypt.php

<?php
/*
  bcrypt class for PHP 5.3 and above.
  An implentation by Marco Arment.
  It uses Blowfish hashing.
  
  Simplified by David Gasperoni <mcdado@gmail.com>.
  
  Forked from Marco Arment <me@marco.org>.
  This code is released in the public domain.
  
  THERE IS ABSOLUTELY NO WARRANTY.
  
  Usage example:

  // In a registration or password-change form:
  $hash_for_user = Bcrypt::hash($_POST['password']);

  // In a login form:
  $is_correct = Bcrypt::check($_POST['password'], $stored_hash_for_user);
  
*/

class Bcrypt
{
    const DEFAULT_WORK_FACTOR = 8;

    public static function hash($password, $work_factor = 0)
    {
        if ( version_compare(PHP_VERSION, '5.3') < 0 ) {
            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 = '$2a$' .
            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)
    {
        if ( version_compare(PHP_VERSION, '5.3') < 0 ) {
          throw new Exception('Bcrypt requires PHP 5.3 or above');
        }

        return crypt($password, $stored_hash) == $stored_hash;
    }

}
?>