PHP Version Upgrade

Upgrade one minor version at a time. Never skip versions -- each step surfaces deprecations that become errors in the next release. Every upgrade follows the same cycle: audit, automate, test, deploy.

Core Principles

Principle	Meaning
Changelog first	Before any upgrade, search the web for the official PHP migration guide (php.net/migration) or ask the user for the changelog -- never rely on static knowledge alone
One version at a time	Upgrade 8.1 -> 8.2 -> 8.3 -> 8.4 sequentially -- skipping versions makes it impossible to isolate breakage
Fix deprecations before upgrading	Deprecations in version N become errors in version N+1 -- treat them as mandatory fixes
Automate first, manual second	Run Rector and PHPCompatibility before touching code by hand -- they catch 80%+ of required changes
Prove it with tests	Never consider an upgrade complete without a passing test suite on the target version
Pin your platform	Set config.platform.php in composer.json to match your lowest deployment target

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Upgrade Process Overview

Phase 0: Read the Changelog

Before touching any code, obtain the actual changelog for the target PHP version:

1. Search the web for PHP X.Y migration guide (e.g., PHP 8.4 migration guide php.net)
2. Or ask the user to provide the changelog / release notes
3. Read the official migration page at https://www.php.net/manual/en/migrationXY.php

This is non-negotiable. Each version has unique changes that static skill knowledge cannot fully capture.

Phase 1: Audit

Before changing any code, understand the scope of the upgrade.

1. Run PHPCompatibility against the target version to identify incompatible code
2. Run php-parallel-lint with the target PHP binary to catch syntax errors
3. Run composer outdated to check if all dependencies support the target version
4. Review the official migration guide at php.net for the target version
5. Check PHP extensions for compatibility (ext-intl, ext-mbstring, etc.)

Phase 2: Automate

Use Rector to handle the bulk of code transformations automatically.

// rector.php
use Rector\Config\RectorConfig;

return RectorConfig::configure()
    ->withPaths([__DIR__ . '/src', __DIR__ . '/tests'])
    ->withPhpSets(php84: true);  // adjust to target version

Always dry-run first:

vendor/bin/rector process --dry-run

Review changes, then apply:

vendor/bin/rector process

Commit Rector changes separately from manual fixes for clean git history.

Phase 3: Update Dependencies

1. Update composer.json with the new PHP version constraint: "php": ">=8.4"
2. Update config.platform.php to match the target version
3. Run composer update and resolve conflicts
4. Update PHP extensions as needed

Phase 4: Test and Deploy

1. Run the full test suite under the new PHP version
2. Run static analysis (PHPStan/Psalm)
3. Deploy to staging and verify
4. Monitor production logs for deprecation notices after deployment

See Upgrade Process Reference for detailed tool configuration, CI pipeline setup, and Docker considerations.

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Tools

Tool	Purpose	When to Use
Rector	Automated AST-based code transformation	First step after auditing -- handles most mechanical changes
PHPCompatibility	PHP_CodeSniffer ruleset for cross-version compatibility	Audit phase -- identifies all incompatible code before you start
php-parallel-lint	Parallel syntax checking (~20x faster than serial)	Audit phase -- catches syntax errors under the new version
PHPStan/Psalm	Static analysis	Verification phase -- catches type errors after transformation
symfony/phpunit-bridge	Deprecation summary in test output	Ongoing -- monitors deprecation count during upgrades

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Breaking Changes by Version

Transition	Key Breaking Changes
8.0 -> 8.1	Fibers introduced, enums added, readonly properties, intersection types, never return type
8.1 -> 8.2	Dynamic properties deprecated (use #[AllowDynamicProperties] temporarily), $GLOBALS access restrictions, readonly classes, disjunctive normal form types
8.2 -> 8.3	Typed class constants, json_validate() added, #[Override] attribute, Randomizer additions, date/time exception changes
8.3 -> 8.4	Implicit nullable types deprecated (function foo(string $bar = null) must become ?string $bar = null), property hooks, asymmetric visibility, new without parentheses deprecated for no-arg constructors, DOM extension namespace changes

See Version Changes Reference for complete per-version details with code examples.

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Common Pitfalls

Pitfall	Why It Hurts	Prevention
Skipping versions	Cannot isolate which changes broke what	Always upgrade one version at a time
Ignoring deprecation warnings	Deprecations become fatal errors in the next version	Fix all deprecations before upgrading
Not checking dependencies	Third-party packages may not support the target version	Run composer outdated and check support before starting
Using --ignore-platform-reqs	Bypasses safety checks, causes runtime errors	Never use it -- fix the actual constraints instead
Not pinning config.platform.php	Local PHP differs from production, causing install mismatches	Always set it to match production
Large unreviewed Rector runs	Rector can make incorrect transformations in edge cases	Always dry-run first, review changes, run on small batches
Forgetting PHP extensions	Extensions change behavior or get deprecated between versions	Audit all required extensions before upgrading

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Quick Reference: Upgrade Checklist

• Review php.net migration guide for target version
• Run PHPCompatibility scan against target version
• Run php-parallel-lint with target PHP binary
• Verify all Composer dependencies support target version
• Configure and run Rector with target version set (dry-run first)
• Review and commit Rector changes
• Apply manual fixes for remaining issues
• Update composer.json PHP constraint and config.platform.php
• Run composer update
• Run full test suite on target PHP version
• Run static analysis (PHPStan/Psalm)
• Deploy to staging and verify
• Deploy to production and monitor logs

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Reference Files

Reference	Contents
Upgrade Process	Detailed tool configuration, CI pipeline setup, Docker strategy, and step-by-step commands
Version Changes	Per-version breaking changes, new features, and deprecations with code examples (PHP 8.0 through 8.4)

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Integration with Other Skills

Situation	Recommended Skill
Upgrading Symfony alongside PHP	Use the symfony-upgrade skill in frameworks/symfony/
Updating Composer dependencies after PHP upgrade	Use the composer-dependencies playbook skill
Modernizing PHP code patterns (DTOs, enums, strict types)	Install php-modernization from dirnbauer/webconsulting-skills or netresearch/php-modernization-skill
Running static analysis after upgrade	Install knowledge-virtuoso from krzysztofsurdy/code-virtuoso for testing strategies