Predefined Constants
Stream Contexts
Streams
PHP Manual

# 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.