public function addUser($user_type, $username, $password)
    {
        // check if user name exists
        $usercount_sql = <<<EOD
\tSELECT
\t\tCOUNT(*)
\tFROM
\t\t`sarah`.`users`
\tWHERE
\t\t`username`='{$username}'
EOD;
        $usercount_data = mysql_query($usercount_sql) or die(mysql_error());
        $usercount_row = mysql_fetch_array($usercount_data);
        if ($usercount_row['COUNT(*)'] == 0) {
            $hashing = AuthManager::createNewHash($password);
            $salt = $hashing['salt'];
            $hash_algorithm = $hashing['hash_algorithm'];
            $hash = $hashing['hash'];
            $salt_database = $hashing['salt_database'];
            $sql = <<<EOD
\tINSERT INTO
\t\t`sarah`.`users` (
\t\t\t`user_type`,
\t\t\t`username`,
\t\t\t`hash_algorithm`,
\t\t\t`salt`,
\t\t\t`password`
\t\t) VALUES (
\t\t\t'{$user_type}',
\t\t\t'{$username}',
\t\t\t'{$hash_algorithm}',
\t\t\t'{$salt_database}',
\t\t\t'{$hash}'
\t\t)
EOD;
            return mysql_query($sql) or die(mysql_error());
        } else {
            return null;
        }
    }