public function setMatchMode($mode)
    {
        assert($mode == SPH_MATCH_ALL
            || $mode == SPH_MATCH_ANY
            || $mode == SPH_MATCH_PHRASE
            || $mode == SPH_MATCH_BOOLEAN
            || $mode == SPH_MATCH_EXTENDED
            || $mode == SPH_MATCH_FULLSCAN
            || $mode == SPH_MATCH_EXTENDED2);
        $this->_mode = $mode;

        return $this;
    }