Add checkstyle formatter

composer.json

@@ -38,6 +38,9 @@
         "symplify/easy-coding-standard": "^6.0",
         "thecodingmachine/phpstan-strict-rules": "^0.11.0"
     },
+    "suggest": {
+        "ext-simplexml": "*"
+    },
     "autoload-dev": {
         "psr-4": {
             "Tests\\": "tests/"

src/Application/Console/Formatters/Checkstyle.php

@@ -0,0 +1,82 @@
+<?php
+
+declare(strict_types=1);
+
+namespace NunoMaduro\PhpInsights\Application\Console\Formatters;
+
+use Exception;
+use NunoMaduro\PhpInsights\Application\Console\Contracts\Formatter;
+use NunoMaduro\PhpInsights\Domain\Contracts\HasDetails;
+use NunoMaduro\PhpInsights\Domain\Details;
+use NunoMaduro\PhpInsights\Domain\Insights\Insight;
+use NunoMaduro\PhpInsights\Domain\Insights\InsightCollection;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * @internal
+ */
+final class Checkstyle implements Formatter
+{
+    /** @var OutputInterface */
+    private $output;
+
+    public function __construct(InputInterface $input, OutputInterface $output)
+    {
+        $this->output = $output;
+    }
+
+    /**
+     * Format the result to the desired format.
+     *
+     * @param InsightCollection $insightCollection
+     * @param string $dir
+     * @param array<string> $metrics
+     */
+    public function format(
+        InsightCollection $insightCollection,
+        string $dir,
+        array $metrics
+    ): void
+    {
+        $checkstyle = new \SimpleXMLElement('<checkstyle/>');
+
+        foreach ($metrics as $metricClass) {
+            /** @var Insight $insight */
+            foreach ($insightCollection->allFrom(new $metricClass()) as $insight) {
+                if (! $insight instanceof HasDetails || ! $insight->hasIssue()) {
+                    continue;
+                }
+
+                /** @var Details $detail */
+                foreach ($insight->getDetails() as $detail) {
+                    $fileName = $this->getFileName($detail, $dir);
+
+                    if (isset($checkstyle->file) && (string) $checkstyle->file->attributes()['name'] === $fileName) {
+                        $file = $checkstyle->file;
+                    } else {
+                        $file = $checkstyle->addChild('file');
+                        $file->addAttribute('name', $fileName);
+                    }
+
+                    $error = $file->addChild('error');
+                    $error->addAttribute('severity', 'error');
+                    $error->addAttribute('source', $insight->getTitle());
+                    $error->addAttribute('line', $detail->hasLine() ? (string) $detail->getLine() : '');
+                    $error->addAttribute('message', $detail->hasMessage() ? $detail->getMessage() : '');
+                }
+            }
+        }
+
+        $this->output->write((string) $checkstyle->asXML());
+    }
+
+    private function getFileName(Details $detail, string $dir): string
+    {
+        if ($detail->hasFile()) {
+            /** replacement is necessary because relative paths are needed */
+            return str_replace($dir . '/', '', $detail->getFile());
+        }
+        return '';
+    }
+}

src/Application/Console/Formatters/FormatResolver.php

@@ -20,6 +20,7 @@ final class FormatResolver
     private static $formatters = [
         'console' => Console::class,
         'json' => Json::class,
+        'checkstyle' => Checkstyle::class,
     ];
 
     public static function resolve(