In a post on Lukas Smith’s blog, he explains the solid reasons why the PHP core developers aren’t just rewriting PHP from scratch to clean it up:
Just today I got yet another email asking me why we do not rewrite PHP to get rid of all the cruft and past mistakes. These kinds of emails obviously have become more frequent since the namespace backslash decision.
He goes into depth on the business and real world reasons why rewriting PHP would be a bad idea overall and a “serious effort”. The full article is a great read.