It's like walking on minefields with very different "mine densities"; when using something stricter, you would have one mine per acre, with PHP you would have ten.
For the longest time the language had been developed with this mentality that it's okay to continue running if something broke, that it's better to print out something than to do nothing and bail out.
Which means that for things to run reliably, you have to write very defensive code that checks everything you can think of. Which is probably a good idea with any language, but I find that old PHP requires much more of this.
Thankfully, they've been changing that over the past decade while still maintaining decent compatibility with old code. I just recently finished porting a pretty large project (~2 mil SLoC) from the ten year old 5.6 to the currently latest 8.4, and it's been pretty painless. The only things that broke were those that were never actually properly implemented and worked by pure chance.