It is evolved indeed, but is there any language designer / guru who ever come close to exploring its good parts (like Douglas Crockford did with Javascript)? I don't think so...
Well, first I think it's irrelevant with what we were saying. Secondly, I doubt there will ever be such a document toward PHP, since it wasn't conceived with a linguistic background (Rasmus Lerdorf wanted a pragmatic dynamic layer on top of C code without resorting to perl). There's no special abstraction principle in PHP, as opposed to prototyping, which (after a quick wikicheck) is not a core abstraction in mainstream languages and could benefit from a book explaining how to think in it. PHP is more and more aiming toward mainstream class based OOP, and everything has been said about it but I can be wrong.