Adds filtername to the list of filters attached to stream. This filter will be added with the specified params to the beginning of the list and will therefore be called first during stream opperations. To add a filter to the end of the list, use stream_filter_append().
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_append()