public function generateUuid() {
        if (is_readable(self::UUID_SOURCE)) {
            $uuid = trim(file_get_contents(self::UUID_SOURCE));
        } elseif (function_exists('mt_rand')) {
            /**
             * Taken from stackoverflow answer, possibly not the fastest or
             * strictly standards compliant
             * @link http://stackoverflow.com/a/2040279
             */
            $uuid = sprintf('%04x%04x-%04x-%04x-%04x-%04x%04x%04x',
                // 32 bits for "time_low"
                mt_rand(0, 0xffff), mt_rand(0, 0xffff),

                // 16 bits for "time_mid"
                mt_rand(0, 0xffff),

                // 16 bits for "time_hi_and_version",
                // four most significant bits holds version number 4
                mt_rand(0, 0x0fff) | 0x4000,

                // 16 bits, 8 bits for "clk_seq_hi_res",
                // 8 bits for "clk_seq_low",
                // two most significant bits holds zero and one for variant DCE1.1
                mt_rand(0, 0x3fff) | 0x8000,

                // 48 bits for "node"
                mt_rand(0, 0xffff), mt_rand(0, 0xffff), mt_rand(0, 0xffff)
            );
        } else {
            // chosen by dice roll, guaranteed to be random
            $uuid = '4';
        }
        return $uuid;
    }