A simple proof of concept of a StackPHP-style implementation of the PSR-7 "immutable" variant.
To download it, git clone and run composer install to grab dependencies.
To use, edit the .htaccess file and index.php to change your base path as appropriate, then try hitting these paths:
Give feedback on the FIG mailing list. It's not impossible this will turn into a for-reals codebase later but at the moment it's just one-day a proof of concept.