The PHP Streams API introduces a unified approach to the handling of files and sockets in PHP extension. Using a single API with standard functions for common operations, the streams API allows your extension to access files, sockets, URLs, memory and script-defined objects. Streams is a run-time extensible API that allows dynamically loaded modules (and scripts!) to register new streams.
The aim of the Streams API is to make it comfortable for developers to open files, urls and other streamable data sources with a unified API that is easy to understand. The API is more or less based on the ANSI C stdio family of functions (with identical semantics for most of the main functions), so C programmers will have a feeling of familiarity with streams.
The streams API operates on a couple of different levels: at the base level, the API defines php_stream objects to represent streamable data sources. On a slightly higher level, the API defines php_stream_wrapper objects which "wrap" around the lower level API to provide support for retrieving data and meta-data from URLs.
Streams can be cast (converted) into other types of file-handles, so that they can be used with third-pary libraries without a great deal of trouble. This allows those libraries to access data directly from URL sources. If your system has the fopencookie() function, you can even pass any PHP stream to any library that uses ANSI stdio!