"It's the PHP 5.3 generation with lots of automagical things and huge full stack frameworks that will be a problem." I tend to disagree there, actually, for one of the other reasons you stated: "Projects with test coverage (unit tests, integration tests, all kinds of tests) will fare best, because it's much more likely that any issues will be found before it's a live environment problem." Most full stack frameworks fall under that latter bullet point, and have copious tests. We found that prepping both ZF1 and ZF2 for PHP 7 was relatively easy due to the number of tests; there were very few places where code was incompatible, and the tests helped find those few spots.