public static function compare_purge_tokens($tokena, $tokenb) {
        if ($tokena === $tokenb) {
            // There is an exact match.
            return 0;
        }

        // The token for when the cache was last invalidated.
        list($atime) = explode('-', "{$tokena}-", 2);

        // The token for this cache.
        list($btime) = explode('-', "{$tokenb}-", 2);

        if ($atime >= $btime) {
            // Token A is newer.
            return 1;
        } else {
            // Token A is older.
            return -1;
        }
    }