While I may not agree to all PSRs (Psr7,Psr15,Psr17 for those of you whom are interested), I really believe standardising how streams work between frameworks and applications is something that PHP-FIG would excel at.
Anyway, I have created a stream-interop group based on an old project of mine. I am looking for developers and supporters alike to help build it. I feel like this is an abstraction that is desperately needed in PHP land. The original interfaces can be found here (including tests and an a concrete implementation): https://github.com/meekframework/stream
The new community-focused group can be found here: https://github.com/stream-interop/stream-interop
My hope is that it will one day be mature enough to make it to a PSR.