function filter($key=NULL,$func=NULL) {
        if (!$key)
            return array_keys($this->filter);
        if (!$func)
            return $this->filter[$key];
        $this->filter[$key]=$func;
    }
function filter($key=NULL,$func=NULL) {
        if (!$key)
            return array_keys($this->filter);
        if (!$func)
            return $this->filter[$key];
        $this->filter[$key]=$func;
    }
function filter ( $ key = NULL , $ func = NULL ) { if ( ! $ key ) return array_keys ( $ this -> filter ) ; if ( ! $ func ) return $ this -> filter [ $ key ] ; $ this -> filter [ $ key ] = $ func ; }
register token filter
@param string $key
@param string $func
@return array
register token filter