story
The difficulty of upgrade for PHP didn't come from the devs having to redo / revalidate parts of their code for little to no immediate benefit (like in Python's case) but in the shared hosting companies (un)willingness to make the upgrade when the scripts their users wanted (phpbb, wordpress, ...) worked fine with PHP 4.
And that's why these various projects agreed to a common date of "end of support" for PHP 4, sending a message to hosting companies to either upgrade, or not be able to deploy wordpress and co anymore