Add asynchronous fetch support

[Bilge]

Add asynchronous fetch support

[Bilge]

The only non-blocking network facility in PHP 7 is the raw socket.

This function works for any stream that supports non-blocking mode (currently, regular files and socket streams).

This has also been independently confirmed by async expert and internals contributor, @bwoebi.

This means that each network protocol has to be implemented using raw socket programming which, needless to say, is extremely laborious and error prone. All the major fledgling async frameworks (React, amphp, icicle.io) provide an HTTP client implementation but they have not been battle tested in a wide variety of real-world scenarios like the PHP stream wrapper and may contain bugs. Moreover, they do not provide other protocols found in PHP such as SOAP.

Given the current level of maturity of async frameworks, I do not think it is practical to resolve this issue until async primitives are available in the PHP language or a wider range of async network protocols are supported.

[Bilge]

It seems amphp is reaching a reasonable level of maturity, however the latest iteration supports PHP 7 only, so this issue may be on hold until Porter has dropped support for earlier PHP versions.

[Bilge]

This has been merged with #54.