Stream Filters
A filter is a final piece of code which may perform
operations 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_filter_register() or in an extension using the
API Reference in Working with streams. To access the list of currently
registered filters, use stream_get_filters().
Predefined Constants
