There are breaking changes from 4->5->7->(less so)8
This is why so many basic php web hosts continue to offer older php versions because the old code will not work and why paid projects like cloudlinux hardened php still exists. Just in the last 2 years they finally ended security support for php4 on the hardened php project.
So to explain how it worked, it wasn’t as sudden as python, but the problem areas have been depreciated and removed as php has evolved.
This is why so many basic php web hosts continue to offer older php versions because the old code will not work and why paid projects like cloudlinux hardened php still exists. Just in the last 2 years they finally ended security support for php4 on the hardened php project.
So to explain how it worked, it wasn’t as sudden as python, but the problem areas have been depreciated and removed as php has evolved.