I find it hard to trust a language that changes so much between minor versions. If I am wrong please tell me where so I can stop my grief at work and finally upgrade the PHP version we use.
Is that true?
There are a few edge cases out there of course. Maybe your code base has basically been in stasis since the early days of PHP 5.x, may not even be using composer or namespaces, relies on tons of abandoned or horribly outdated dependencies, doesn't have good test coverage, etc., and then suddenly someone gets a bee in their bonnet about PHP 7.x and wants to upgrade. That's often not going to work out very well, but that's not really a PHP 7.x (or even a PHP thing). :) If you've been maintaining the code, have tests, no ancient vulnerable libraries, using current framework versions, etc, the process is going to be easy. If you're trying a big bang upgrade straight from the medieval period, not so much!
So maybe upgrading PHP itself might not be that bad, something I can't tell you for sure, but in my experience nobody uses vanilla PHP, they all wrap it in frameworks, and those must all be carefully upgraded too.