PHP has changed a lot over time. It moved from a C-style procedural language to very Java-esque object oriented language. "PHP" hasn't been "PHP" for a long time already.
It's always hard to do large language changes, but it's not impossible. E.g. consider the change that PHP did from version 4 to 5. That was a pretty major paradigm shift, with lots of broken code, but it worked out well in the end :)