A legacy PHP application is often the software equivalent of a building that has been extended a dozen times: it works, the business depends on it, and nobody wants to touch it. Old PHP versions, no tests, mixed concerns, and years of accumulated shortcuts make every change risky. The good news is that legacy PHP modernisation does not require a big-bang rewrite, which is usually the riskiest option of all. This guide lays out a safer, incremental path.
TL;DR
A full rewrite is the most tempting and most dangerous option; incremental modernisation is safer and delivers value sooner
Start by assessing the code, upgrading to a supported PHP version, and adding a safety net of tests around critical behaviour
Introduce Composer, autoloading, and modern structure, then refactor toward clean separation of concerns
