What do you mean? Backwards compatibility in PHP is amazing. I have several large projects that I've been working on for 15+ years, and upgrading to newer PHP versions has been a dream.
What kind of things are you having problems with in regards to backwards compatibility?
While "abandon those projects" doesn't help someone who developped something with it and need to keep them updated, the fact is that you can just keep using the older version of the library while you do your update it works just fine on newer PHP, AND pretty much any decent sized projects doing such a switch has a large transition period where they support both, AND tooling to automate such upgrade exists AND we should be very happy about removing code from comments.
Most big projects offer a large transition period. And their "older" version works just fine on latest PHP versions anyway, so you have the time to do your upgrade. And the tooling to make such upgrade is very stable and well working, rector does 99% of the job.
I love PHP, and think updates over the last several years have been relatively painless. But it doesn't feel like it's always been that way over the last 15 years.
What kind of things are you having problems with in regards to backwards compatibility?