Psr7 Http Message, why immutable?

Question

I am looking at the PSR-7 interfaces and thinking of a way on how to implement them.

I\'ve also been reading this blog post. Apparently the objects that implement the

Answer 1

I suggest you to read this document, where all the design choices are explained in detail.

In particular you should read the Why value objects? and the New instances vs returning $this sections.

The key points are the following:

In essence, modeling HTTP messages as value objects ensures the integrity of the message state, and prevents the need for bi-directional dependencies, which can often go out-of-sync or lead to debugging or performance issues.

and

These operations can be accomplished with value objects as well, with a number of benefits:

• The original request state can be stored for retrieval by any consumer.
• A default response state can be created with default headers and/or message body.

If you want to dig deeper, I'd recommend to look in the history of the fig mailing list (you can find it here), where there was a lot of discussion regarding the immutability of the objects