feat(php): Add PHP SDK implementation

[clemensv]

Summary

Add a PHP SDK for JSON Structure schema and instance validation.

Background

PHP powers approximately 77% of websites with a known server-side language. With Laravel, Symfony, and WordPress dominating web development, a PHP SDK provides schema validation for the massive PHP ecosystem including:

• Laravel/Symfony API validation
• WordPress plugin data validation
• Composer package ecosystem
• Legacy PHP application modernization

Implementation Guide

Follow the SDK Guidelines document: SDK-GUIDELINES.md

This document contains:

• Required components (SchemaValidator, InstanceValidator, ErrorCodes, JsonSourceLocator)
• Complete type system reference (34 primitive types + compound types)
• Keywords reference (what to implement, what NOT to implement)
• Error codes reference (assets/error-messages.json)
• Test assets for conformance testing
• Conformance checklist

Requirements

Core Features

• Schema Validation: Validate JSON Structure schema documents for conformance
• Instance Validation: Validate JSON instances against JSON Structure schemas
• Error Reporting: Line/column information for validation errors (when source JSON available)
• Full Type Support: All 34 primitive and compound types from JSON Structure Core v0

Technical Requirements

• PHP 8.1+ (for modern type system, enums, readonly properties)
• Composer for distribution
• Use json_decode() / json_encode() for JSON parsing
• PSR-4 autoloading
• PHPUnit for testing
• No framework dependencies (pure PHP, works with any framework)

Package Structure

php/
 composer.json
 README.md
 phpunit.xml
 src/
    JsonStructure/
        SchemaValidator.php
        InstanceValidator.php
        Types.php
        ErrorCodes.php
        JsonSourceLocator.php
        ValidationResult.php
 tests/
     SchemaValidatorTest.php
     InstanceValidatorTest.php
     TestAssets.php

CI/CD

• GitHub Actions workflow with PHP matrix (8.1, 8.2, 8.3)
• Test on ubuntu-latest
• Integration with shared test-assets/ for cross-SDK validation

Reference Materials

• SDK Guidelines: SDK-GUIDELINES.md - Start here
• Error Messages: assets/error-messages.json
• Test Assets: test-assets/
• Meta-schemas: meta/
• Reference implementations: go/, rust/, typescript/, python/

Acceptance Criteria

• All test-assets pass validation
• CI workflow passes on PHP 8.1, 8.2, 8.3
• README with installation and usage examples
• Conformance checklist completed (see SDK-GUIDELINES.md)
• Published to Packagist (or ready for publication)