More specifically, how many implementations of PSR-7 ServerRequestInterface exist?
Initially, it appears the answer might be as many as three dozen.
But increasingly it looks like the real answer, to a first approximation, is “zero.”
To see for yourself, clone my PSR-7 Immutability Testing repository and run the tests, which cover the three most popular PSR-7 implementations.
Immutability is very difficult to implement – at least, not without some serious restrictions, none of which are specified by PSR-7 ServerRequestInterface.The more I work with it, the more I begin to think that it is not fully implementable-as-specified in the first place.
UPDATE: Apparently I was on to something when I suggested that PSR-7 cannot be implemented as written. Sara Golemon (who has forgotten more about PHP internals than I will ever know) points out, "It is not technically implementable. And thus you're technically correct, the best kind of correct." (Archived for posterity at https://archive.is/BnkGb .)
UPDATE 2: Reddit commenters point out that the ServerRequestInterface::getAttributes() method states "Attributes will be application and request specific, and CAN be mutable." This seems at odds with the withAttribute() method, which requires that the object remain immutable. At the best, this seems like an oversight in the spec, and at the worst it is an internal contradiction.