actually in future versions symfony will move mostly to php based configs instead of yamk, xml. at least parts of internal configs which were xml now are rewritten to php

Which is a sad choice on it's own

Is it? Why is that?

First because you don't really have completion or templating over its content: you mostly do copy/pasting or guessing what you have to put in there.

Second because they are using all kind of tricks to add variables into it (ENV, ...): the configuration is often dynamic by nature or by re-use.

Third because it's only strings, which is prone to mistakes: they are having a huge lot of code to check those configurations.