PHP Debugging

php-debugging

When to use

Use this skill when:

• Setting up or troubleshooting Xdebug
• Debugging PHP code with breakpoints
• Investigating performance issues
• Running code coverage
• Helping users configure their IDE for debugging

This skill extends php-coder and php.

Procedure: Debug with Xdebug

1. Detect the project's debug setup — check docker-compose.yml / compose.yaml for Xdebug containers.
2. Check Dockerfile — look for a dev-xdebug build stage or Xdebug installation.
3. Check NGINX config — look for header-based routing to a debug container.
4. Read project docs — check Docs/XDEBUG_SETUP.md or docs/ for setup instructions.
5. Check .env — look for DOCKER_XDEBUG_MODE and DOCKER_XDEBUG_PORT.

Dual-container architecture

Many projects use two PHP containers for optimal performance:

NGINX routes requests based on HTTP headers:

• No debug header → fast container (no Xdebug)
• Debug header present → Xdebug container

Debug headers

Important: Use hyphens, not underscores. XDEBUG_SESSION does NOT work — use XDEBUG-SESSION.

Verify routing

Check the X-PHP-Backend response header to confirm which container handled the request:

# Normal request
curl -I http://localhost:8002/  # → X-PHP-Backend: *-php:9000

# Debug request
curl -I -H "X-Xdebug-Enable: 1" http://localhost:8002/  # → X-PHP-Backend: *-php-xdebug:9000

Xdebug configuration

Environment variables

DOCKER_XDEBUG_MODE=develop,debug,coverage   # Xdebug modes
DOCKER_XDEBUG_PORT=9003                      # IDE listens on this port

Xdebug modes

PHP-FPM on-demand (Xdebug container)

The Xdebug container typically uses pm = ondemand to save resources:

• Workers start only when a debug request arrives
• Idle workers terminate after 60s
• Reduces memory usage by ~60-70% when not debugging
• Unlimited request timeout for breakpoint sessions

IDE setup

PhpStorm

1. Debug port: Settings → PHP → Debug → Port: 9003
2. Enable listening: Click "Start Listening for PHP Debug Connections" (phone icon)
3. Server config: Settings → PHP → Servers:
   • Host: localhost, Port: 80 (internal container port, not host port)
   • Enable path mappings: local project root → /var/www/html
4. Browser extension: Install "Xdebug helper", set IDE key to PHPSTORM

VS Code

{
  "name": "Listen for Xdebug",
  "type": "php",
  "request": "launch",
  "port": 9003,
  "pathMappings": {
    "/var/www/html": "${workspaceFolder}"
  }
}

Debugging workflow

1. Start listening in IDE (PhpStorm: green phone icon)
2. Set breakpoints in code
3. Send request with debug header (browser extension, Postman, or curl)
4. IDE breaks at breakpoint → inspect variables, step through code
5. Check X-PHP-Backend header if debugging doesn't trigger

CLI debugging (Artisan commands, tests)

For debugging Artisan commands or tests, enter the Xdebug container:

make console-xdebug    # Enter Xdebug container
php artisan your:command   # Xdebug connects to IDE automatically

Verifying CLI fixes

After a fix, verify the command without re-attaching the debugger:

# Run the command, capture exit code + command output
php artisan your:command; echo "exit code: $?"

# Pest CLI assertion — expectsOutput / artisan test
vendor/bin/pest --filter='ProcessInvoicesCommand'

Assert on the exit code and command output; never trust "looks fine" from breakpoint inspection alone.

Troubleshooting

Container management

make console              # Fast PHP container (no Xdebug)
make console-xdebug       # Xdebug container
make rebuild-php-xdebug   # Rebuild Xdebug container only
make rebuild-php-all      # Rebuild both PHP containers

Filter noisy debug output

tail -f /tmp/xdebug.log and docker logs produce far too much data to read line-by-line. Filter with rg/grep for the relevant event:

# Only connection / breakpoint events
rg --color=never 'Connect|Step|breakpoint' /tmp/xdebug.log

# Only this request's frames in the laravel log
docker compose logs php-xdebug | rg --color=never "$REQUEST_ID"

What NOT to do

• Do not leave Xdebug enabled in production containers.
• Do not use underscores in debug headers (XDEBUG_SESSION fails — use XDEBUG-SESSION).
• Do not set PhpStorm server port to the host port (e.g. 8002) — use the internal port (80).
• Do not run performance benchmarks against the Xdebug container.
• Do not forget to check path mappings when breakpoints are silently skipped.

Output format

1. Xdebug configuration or debugging session setup
2. Root cause identified with evidence from debugger output

Gotcha

• Xdebug runs in a separate container — don't confuse the fast container (port 80) with the debug container (port 8080).
• The model tends to suggest dd() or var_dump() — they're forbidden by PHPStan config. Use Xdebug breakpoints.
• Step-debugging over HTTP requires the XDEBUG_SESSION cookie/header — without it, breakpoints don't trigger.

Do NOT

• Do NOT leave breakpoints or debug code in committed files.
• Do NOT use var_dump() or dd() — use Xdebug breakpoints.
• Do NOT debug in the fast container — switch to the Xdebug container.

Clarification guard — ambiguous repro → ask

If the bug repro is unclear (which request, which user, which env, which input shape?), do not start placing breakpoints at random. Ask the user for the failing payload, the exact route, or the Sentry trace. Setting breakpoints based on a guessed flow wastes a session.

Auto-trigger keywords

• Xdebug
• PHP debugging
• breakpoint
• step debugging

Known Laravel bug patterns (from bug-analyzer)

• N+1 queries hidden in loops or accessors
• Missing ->fresh() after update when using same instance
• Eloquent lazy loading in queued jobs (serialization issues)
• now() timezone mismatches
• Missing FK constraints allowing orphaned records
• DB::transaction() with external side effects (emails, API calls)
• Model events firing during seeding or migration
• Off-by-one errors in pagination or date ranges
• Silent exception swallowing (catch (\Exception $e) {})
• Floating point comparison for money/quantities