diff --git a/composer.json b/composer.json
index 9df9728..db99ff7 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,9 @@
   "prefer-stable": true,
   "require": {
     "php": "^7.0",
+    "ext-dom": "*",
     "ext-json": "*",
+    "ext-simplexml": "*",
     "juliangut/http-exception": "^0.1.1",
     "psr/log": "^1.0",
     "slim/slim": "^3.5",
diff --git a/src/ExceptionManager.php b/src/ExceptionManager.php
index 71a3f1f..c56b09b 100644
--- a/src/ExceptionManager.php
+++ b/src/ExceptionManager.php
@@ -107,12 +107,7 @@ public function addHandler($exceptionTypes, ExceptionHandler $handler)
             $exceptionTypes = [$exceptionTypes];
         }
 
-        $exceptionTypes = \array_filter(
-            $exceptionTypes,
-            function ($exceptionType): bool {
-                return \is_string($exceptionType);
-            }
-        );
+        $exceptionTypes = \array_filter($exceptionTypes, '\is_string');
 
         foreach ($exceptionTypes as $exceptionType) {
             $this->handlers[$exceptionType] = $handler;
diff --git a/src/Handler/ExceptionHandler.php b/src/Handler/ExceptionHandler.php
index ca0177d..bcc2f74 100644
--- a/src/Handler/ExceptionHandler.php
+++ b/src/Handler/ExceptionHandler.php
@@ -70,12 +70,7 @@ public function addFormatter(ExceptionFormatter $formatter, $contentTypes = null
             $contentTypes = [$contentTypes];
         }
 
-        $contentTypes = \array_filter(
-            $contentTypes,
-            function ($contentType): bool {
-                return \is_string($contentType);
-            }
-        );
+        $contentTypes = \array_filter($contentTypes, '\is_string');
 
         if (\count($contentTypes) === 0) {
             throw new \RuntimeException(\sprintf('No content type defined for %s formatter', \get_class($formatter)));
diff --git a/src/HttpExceptionAwareTrait.php b/src/HttpExceptionAwareTrait.php
index 39fc0d3..8e72211 100644
--- a/src/HttpExceptionAwareTrait.php
+++ b/src/HttpExceptionAwareTrait.php
@@ -15,6 +15,7 @@
 
 use Jgut\HttpException\HttpException;
 use Jgut\HttpException\InternalServerErrorHttpException;
+use Psr\Container\ContainerInterface;
 use Psr\Http\Message\ResponseInterface;
 
 /**
@@ -117,14 +118,14 @@ protected function getLastError(): array
      */
     private function isFatalError(int $error): bool
     {
-        $fatalErrors = E_ERROR
-            | E_PARSE
-            | E_CORE_ERROR
-            | E_CORE_WARNING
-            | E_COMPILE_ERROR
-            | E_COMPILE_WARNING
-            | E_USER_ERROR
-            | E_STRICT;
+        $fatalErrors = \E_ERROR
+            | \E_PARSE
+            | \E_CORE_ERROR
+            | \E_CORE_WARNING
+            | \E_COMPILE_ERROR
+            | \E_COMPILE_WARNING
+            | \E_USER_ERROR
+            | \E_STRICT;
 
         return ($error & $fatalErrors) !== 0;
     }
@@ -198,9 +199,9 @@ function (array $frame): array {
     /**
      * Enable access to the DI container by consumers of $app.
      *
-     * @return \Psr\Container\ContainerInterface
+     * @return ContainerInterface
      */
-    abstract public function getContainer();
+    abstract public function getContainer(): ContainerInterface;
 
     /**
      * Send the response the client.
diff --git a/src/Whoops/Formatter/Xml.php b/src/Whoops/Formatter/Xml.php
index 40400d4..9514d88 100644
--- a/src/Whoops/Formatter/Xml.php
+++ b/src/Whoops/Formatter/Xml.php
@@ -98,9 +98,9 @@ protected function addDataNodes(\SimpleXMLElement $node, array $data, string $no
     {
         foreach ($data as $key => $value) {
             if (\is_numeric($key)) {
-                $key = $nodeKey . '_' . (string) $key;
+                $key = $nodeKey . '_' . $key;
             }
-            $key = \preg_replace('/[^a-z0-9\-\_\.\:]/i', '_', $key);
+            $key = \preg_replace('/[^a-z0-9\-_.:]/i', '_', $key);
 
             if (\is_array($value)) {
                 $this->addDataNodes($node->addChild($key), $value, $key);
diff --git a/tests/Exception/Stubs/AppStub.php b/tests/Exception/Stubs/AppStub.php
index 3a0cbe7..ad760df 100644
--- a/tests/Exception/Stubs/AppStub.php
+++ b/tests/Exception/Stubs/AppStub.php
@@ -50,7 +50,7 @@ public function __construct(ContainerInterface $container, array $lastError = []
     /**
      * {@inheritdoc}
      */
-    public function getContainer()
+    public function getContainer(): ContainerInterface
     {
         return $this->container;
     }