PHP4 to PHP5 was almost as big of a deal as Python2 to Python3. In fact, while I have PHP5 code that can easily ported to PHP7 -- I still have PHP4 code that will be forever on PHP4.
Because of this, PHP developers strive to avoid this kind of large breakage. Instead it's a smooth stream of deprecation and removals/changes and really large changes (like native unicode strings) have been non-starters.
I might be misguided, but the last time I used PHP was over ten years ago, using PHP3, and we had to name the files my_file.php3 in order to use PHP3. That's the opposite of minimal breakage. On the other hand, my simple PHP3 scripts continue to work to this day, php3 extension and all!
It's more like after PHP 5 the community had learned that lesson, while python didn't have the chance yet.
Given that, I think the PHP community handled the split much better, with the FIG pushing hard for PHP 5.
I think the difference is that most of PHP install base was managed by third parties (hosting providers) and once you lobbied those you basically forced everyone to upgrade. With python most of the install base is proprietary, and thus more fragmented, which makes it much more complicated to lobby for an upgrade.
