A Practical Guide to Migrating Legacy PHP Systems to Modern Stacks

The instinct when facing a decade-old PHP codebase is to rewrite it from scratch. Resist it. Big-bang rewrites fail more often than they succeed, mostly because the old system encodes years of undocumented business logic that only surfaces once you've broken it.

The strangler-fig pattern — named after the vine that grows around a host tree until it replaces it — is a safer alternative. You put a routing layer in front of the legacy system, then migrate functionality piece by piece, routing traffic to the new implementation only once it's verified equivalent.

Start with the parts of the system that change most often, not the parts that are technically worst. High-churn code benefits most immediately from a modern stack, and migrating it first builds momentum and trust with stakeholders watching the project.

Keep the legacy database as the source of truth for as long as possible, and use an anti-corruption layer to translate between old and new data models. Trying to migrate the database and the application logic simultaneously is where most of these projects go sideways.

Budget real time for the unglamorous 20% — session handling, cron jobs, and third-party integrations nobody remembers configuring. These are consistently where legacy migrations lose the most time.