We migrated the codebase of our most ancient system (started in 2008) to PHP8 this May. Millions of lines of code. It mostly consists of PHP5-era code, and ther

We migrated the codebase of our most ancient system (started in 2008) to PHP8 this May. Millions of lines of code. It mostly consists of PHP5-era code, and there's also newer code which is compatible with PHP7. It took 4 weeks of work of 1 dev and 1 QA (including bugfixing). The engineer responsible for the conversion downloaded all existing linters he could find, ran them and fixed code until they stopped complaining. He also he used Rector - software which can semi-automatically update PHP code to newer versions. We had a few non-critical regressions after the release due to the breaking changes, but otherwise I was surprised how smooth the release was. My favorite feature is "constructor property promotion", a lot of classes now take 2 times less code.