However that doesn't work forever, at some point you need to clean up some of the old stuff. That's why a PHP 6 release is planned in the near future (i.e. somewhere in the next three years ^^).
I've since moved on to more well thought out languages, though PHP keeps pulling me back in. I'm currently having to support a CI app that hasn't had a line of code added to it since 2008, and the only thing I can say, even though I appreciate the language and how far it's pushed the web, I'm glad it's no longer my go-to language of choice. Keep PHP5 around for a while so apps like this one can continue on, but PHP6 is _LONG_ overdue.
However due to problems and internal conflicts the effort was stopped a few years ago and there has been no progress since.
If this goal hadn't been stated PHP 5.3+5.4 would probably have been PHP 6 (they have some major new features). Core API cleanup never has been (and it doesn't look like it will soon be) a goal for a future release, unfortunately.