However, if your filter increases the size of the data being returned, you should either call parent::read( $maxlength-$x); where x is the most your filter will grow the size of the data read.
This method is called in response to a request to flush the attached stream (such as with fflush() or fclose()). The closing parameter tells you whether the stream is, in fact, in the process of closing. The default action is to simply call: return parent::flush($closing);, your filter may wish to perform additional writes and/or cleanup calls prior to or directly after a successful flush.