The 8 series has made a lot of little changes that are currently deprecations but if converted to errors as promised in 9 will be the end for lots of legacy cod

The 8 series has made a lot of little changes that are currently deprecations but if converted to errors as promised in 9 will be the end for lots of legacy code without changes: most of the standard library not taking null arguments where it used to, changed/added typehinting for the standard classes affecting anything that extends/implements them, and disallowing dynamic properties on objects are some big ones that come to mind. Once 9 makes some of this mandatory it'll also be much more difficult for a single codebase to work across a wide range of PHP versions. That's possibly even an explicit goal: forcing narrower support from applications and libraries will prod more hosts and end-users to update. Not that these are bad changes, but that the philosophy of extreme compatibility really isn't in force anymore. It's not a Python 2/3 scenario but a gradual constant shift.