There are many wrappers built into PHP by default (See Appendix I), and additional, custom wrappers may be added either within a PHP script using stream_register_wrapper(), or directly from an extension using the API Reference in Chapter 43.
A filter is a final piece of code which may perform opperations on data as it is being read from or written to a stream. Any number of filters may be stacked onto a stream. Custom filters can be defined in a PHP script using stream_register_filter() or in an extension using the API Reference in 43장. To access the list of currently registered filters, use stream_get_filters().