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