Migrating from PHP 5.6/7.x to PHP 8.3: A Practical Roadmap

Why upgrade now

• PHP 7.4 reached end-of-life in November 2022 — no security patches.
• PHP 8.x delivers 15–30% performance gains out of the box (JIT, opcache improvements).
• Modern PHP (typed properties, enums, readonly, first-class callable syntax) reduces bug surface dramatically.
• Most modern libraries no longer support PHP 7.

The 6-phase roadmap

Phase 1 — Audit (1–2 weeks)

• Inventory: PHP version, framework version, all composer dependencies and their PHP requirements.
• Run PHPStan at level 0–3 to baseline current type coverage.
• Identify deprecated APIs with `phan` or `vimeo/psalm`.
• List third-party libraries with no PHP 8 support — these need replacements or forks.

Phase 2 — Stabilize tests (2–4 weeks)

You can't refactor what you can't verify. Before touching PHP versions, get test coverage on the critical paths (auth, payments, data writes). You don't need 90% coverage — you need confidence on the routes that pay the bills.

Phase 3 — Automated rewrites with Rector (1–3 weeks)

Rector applies the bulk of syntax-level migrations automatically. Run it incrementally with focused rule sets:

composer require rector/rector --dev
./vendor/bin/rector process src --set=php80
./vendor/bin/rector process src --set=php81
./vendor/bin/rector process src --set=php82

Phase 4 — Manual fixes & dependency upgrades (2–6 weeks)

• Resolve dependency conflicts in composer.json.
• Replace abandoned packages with modern alternatives.
• Address Rector edge cases — magic methods, dynamic properties (now deprecated), reference returns.
• Update framework (Laravel 5 → 11 is a separate roadmap of its own).

Phase 5 — Staging & soak (1–2 weeks)

Deploy to a production-mirror environment. Run the full test suite, plus synthetic load. Watch error rates, response times, memory usage. Pay attention to type coercion changes — PHP 8 is stricter, and silent bugs in PHP 7 become loud exceptions.

Phase 6 — Production rollout

If you're on Kubernetes or have multiple workers, do a canary: 5% → 25% → 100%. If you're on a single server, schedule a maintenance window — but most modern hosts let you swap PHP versions per host with FPM pools.

Realistic timeline

• Small app (<50k LOC, modern framework): 2–4 weeks.
• Medium app (50–200k LOC, Laravel 5–8): 6–12 weeks.
• Large legacy (200k+ LOC, custom framework or old Symfony): 3–6 months.

Risks people underestimate

• Dynamic properties: PHP 8.2+ deprecates them. Lots of legacy code breaks subtly.
• Stricter type juggling: comparisons like `0 == 'foo'` flip from true to false.
• Date/time defaults change behavior for partial date strings.
• Removed extensions (mcrypt) — if you're still using them for old encrypted data, plan re-encryption.

What you get on the other side

Beyond performance and security: smaller bug count thanks to types, easier hiring (modern engineers refuse legacy projects), and access to the modern PHP ecosystem (Laravel 11+, Symfony 7+, modern testing tools).