chore: Add php-cs-fixer.

.github/workflows/php-cs-fixer.yml

@@ -0,0 +1,23 @@
+name: Check & fix styling
+
+on: [push]
+
+jobs:
+  php-cs-fixer:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+        with:
+          ref: ${{ github.head_ref }}
+
+      - name: Run PHP CS Fixer
+        uses: docker://oskarstark/php-cs-fixer-ga
+        with:
+          args: --config=.php-cs-fixer.dist.php --allow-risky=yes
+
+      - name: Commit changes
+        uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: Fix styling

.php-cs-fixer.dist.php

@@ -0,0 +1,39 @@
+<?php
+
+$finder = Symfony\Component\Finder\Finder::create()
+    ->in([
+        __DIR__ . '/app',
+        __DIR__ . '/tests',
+    ])
+    ->name('*.php')
+    ->ignoreDotFiles(true)
+    ->ignoreVCS(true);
+
+return (new PhpCsFixer\Config())
+    ->setRules([
+        '@PSR12' => true,
+        'array_syntax' => ['syntax' => 'short'],
+        'ordered_imports' => ['sort_algorithm' => 'alpha'],
+        'no_unused_imports' => true,
+        'not_operator_with_successor_space' => true,
+        'trailing_comma_in_multiline' => true,
+        'phpdoc_scalar' => true,
+        'unary_operator_spaces' => true,
+        'binary_operator_spaces' => true,
+        'blank_line_before_statement' => [
+            'statements' => ['break', 'continue', 'declare', 'return', 'throw', 'try'],
+        ],
+        'phpdoc_single_line_var_spacing' => true,
+        'phpdoc_var_without_name' => true,
+        'class_attributes_separation' => [
+            'elements' => [
+                'method' => 'one',
+            ],
+        ],
+        'method_argument_space' => [
+            'on_multiline' => 'ensure_fully_multiline',
+            'keep_multiple_spaces_after_comma' => true,
+        ],
+        'single_trait_insert_per_statement' => true,
+    ])
+    ->setFinder($finder);