PHP is a perfect example of how to undergo major transitions correctly.
In the time that it took Python to go from Python 2 to 3, PHP underwent 5.2 -> 5.3 and 5.6 -> 7.0. 5.3 changed how you write PHP in a fundamental way by introducing namespaces and PSR0 autoloading. Then, 5.6 -> 7.0 cleaned up the parser, resulting in massive speed improvements.
They did this by not breaking the universe in these major updates...or really, any of there updates. Each update would break a few specific things, warn you about a few other things that were going to break in the near future, while giving you ways to shim the old behavior if you _really_ needed it.
They also gave you ample carrots reasons to update - aside from what was already mentioned, my personal favorite update was PHP 5.4 because it introduced short array syntax, made UTF-8 the default charset, introduced traits, and finally put register globals and magic quotes out of their misery...but giving you a shim if you had some ancient project that needed it.
If you're wondering why PHP 6 was abandoned, it was because it was an update in the style of Python 3 by breaking strings everywhere. In retrospect, I think the decision to cancel 6 was the right one. Somehow, I doubt that PHP would've had the influx of data science and AI that saved Python 3.
Hats off to the project - I've long been off of the CGI-style web development narcotic, but the language put food on my table for quite some time, and the people steering the language treated their community well.