/**
    * Save some data in a cache file
    *
    * @param string $data data to put in cache (can be another type than strings if automaticSerialization is on)
    * @param string $id cache id
    * @param string $group name of the cache group
    * @return boolean true if no problem
    * @access public
    */
    function save($data, $id = NULL, $group = '