feat: add php rule pack

rules/php/coding-style.md

@@ -0,0 +1,35 @@
+---
+paths:
+  - "**/*.php"
+  - "**/composer.json"
+---
+# PHP Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with PHP specific content.
+
+## Standards
+
+- Follow **PSR-12** formatting and naming conventions.
+- Prefer `declare(strict_types=1);` in application code.
+- Use scalar type hints, return types, and typed properties everywhere new code permits.
+
+## Immutability
+
+- Prefer immutable DTOs and value objects for data crossing service boundaries.
+- Use `readonly` properties or immutable constructors for request/response payloads where possible.
+- Keep arrays for simple maps; promote business-critical structures into explicit classes.
+
+## Formatting
+
+- Use **PHP-CS-Fixer** or **Laravel Pint** for formatting.
+- Use **PHPStan** or **Psalm** for static analysis.
+- Keep Composer scripts checked in so the same commands run locally and in CI.
+
+## Error Handling
+
+- Throw exceptions for exceptional states; avoid returning `false`/`null` as hidden error channels in new code.
+- Convert framework/request input into validated DTOs before it reaches domain logic.
+
+## Reference
+
+See skill: `backend-patterns` for broader service/repository layering guidance.