With PHP7 looking increasingly stable (relatively speaking, it's still pre-alpha so it's VERY early days and anything could happen!), and work going well on the GoPHP7-ext project to get extensions converted, I have been thinking about the migration guides we'll need to help people upgrade their existing applications. To this end, I took the simplest project I currently have (http://api.joind.in) and gave it a whirl on PHP7, using Rasmus' PHP7 dev box. The result:

Total lines of code change needed to make the @joindin API work on PHP7: zero

—Lorna Mitchell (@lornajane) May 14, 2015

All in all, it wasn't a great study of what kinds of things can go wrong when upgrading projects, because as far as I can tell with the test coverage that we have, it Just Works (TM).

I think the main reason for this success is that this project doesn't use many (any?) dependent libraries. Those libraries are catching up fast to PHP7 but you will probably need to update to the newest versions of everything prior to doing a PHP5 to PHP7 platform upgrade. If your project uses libraries that aren't already working on PHP7, check if they already have a branch for updates and use that in your testing (see also: Use a GitHub Branch as a Composer Dependency). If you can still break it, we love you!! Please immediately file a bug with a replication case and keep trying to break it if/when fixes are supplied. NB: this is open source, you may well need to fix things yourself.

So what kind of project will "Just Work" on PHP7?

• Smaller codebases are more likely to upgrade without issues, they are also much easier to debug!
• Older codebases are also likely to be fine! We're removing the PHP4 style constructor but the majority of those simple PHP4-style MVC applications will probably run fine on PHP7. It's the PHP 5.3 generation with lots of automagical things and huge full stack frameworks that will be a problem.
• Code created with modern component frameworks and running on PHP 5.5 or PHP 5.6 will probably upgrade without much fuss
• 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.

After this failed experiment of what to look out for when upgrading a codebase, I'll be following up with my findings on other upgrades and sharing what to look out for. If you're starting to test how your code runs on PHP7, then I'd be interested to hear how you get on - please leave me a comment.