I often wonder why a PHP killer hasn't yet been created.

I don't mean a Ruby or another vastly different language.

I mean a language similar in approach to PHP, but with the gotcha's removed, some order and consistency introduced into the API, and a few changes to the defaults. Open source, free, and on all major OS's.

check what PyPy did (or is doing with PHP).

Could you please explain it a bit or provide a link?

HipHop its Facebook sponsored just in time compiler for php. HHVM (Hip Hop Virtual Machine). My understanding is its 95%ish compatible with most php. They're working using framework unit tests to get the percent higher. \nhttp://www.hhvm.com/frameworks/

https://github.com/facebook/hhvm