Run the test suite on GitHub Actions across PHP versions

Adapted from XRay's workflow. Tests run on PHP 7.3 through 8.4: the
library supports 7.1+, but PHPUnit 9.6 needs 7.3. There's no committed
composer.lock, so dependencies are resolved per PHP version and the
cache is keyed on composer.json. ext-sockets is enabled so the resolver
test covers hosts-file IPv6 lookups.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/php.yml

@@ -0,0 +1,46 @@
+name: PHP Composer
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        # The library supports PHP 7.1+, but PHPUnit 9.6 needs 7.3 or later
+        php-versions: ["7.3", "7.4", "8.0", "8.1", "8.2", "8.3", "8.4"]
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup PHP, with composer and extensions
+        uses: shivammathur/setup-php@v2 #https://github.com/shivammathur/setup-php
+        with:
+          php-version: ${{ matrix.php-versions }}
+          # sockets lets the resolver read IPv6 entries from the hosts file
+          extensions: mbstring, dom, curl, sockets
+          coverage: pcov
+
+      # Not --strict: the open-ended mf2/mf2 constraint is only a warning
+      - name: Validate composer.json
+        run: composer validate
+
+      - name: Cache Composer packages
+        id: composer-cache
+        uses: actions/cache@v4
+        with:
+          path: vendor
+          key: ${{ runner.os }}-php-${{ matrix.php-versions }}-${{ hashFiles('composer.json') }}
+          restore-keys: |
+            ${{ runner.os }}-php-${{ matrix.php-versions }}-
+
+      # There's no committed composer.lock, so resolve for each PHP version
+      - name: Install dependencies
+        run: composer update --prefer-dist --no-progress
+
+      - name: Run test suite
+        run: composer run-script test