Adds filtername to the list of filters attached to stream. This filter will be added with the specified params to the end of the list and will therefore be called last during stream opperations. To add a filter to the beginning of the list, use stream_filter_prepend().
Note: stream_register_filter() must be called first in order to register the desired user filter to filtername.
See Also: stream_register_filter(), and stream_filter_prepend()