diff --git a/composer.json b/composer.json
index 8a6e9f4..8701a52 100644
--- a/composer.json
+++ b/composer.json
@@ -25,6 +25,8 @@
         "sebastian/comparator": ">=2"
     },
     "suggest": {
+        "ext-json": "To detect JSON responses with the ContentTypePlugin",
+        "ext-libxml": "To detect XML responses with the ContentTypePlugin",
         "php-http/logger-plugin": "PSR-3 Logger plugin",
         "php-http/cache-plugin": "PSR-6 Cache plugin",
         "php-http/stopwatch-plugin": "Symfony Stopwatch plugin"
diff --git a/src/Plugin/ContentTypePlugin.php b/src/Plugin/ContentTypePlugin.php
index f6dcddd..f3944dd 100644
--- a/src/Plugin/ContentTypePlugin.php
+++ b/src/Plugin/ContentTypePlugin.php
@@ -94,11 +94,11 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
         return $next($request);
     }
 
-    /**
-     * @param $stream StreamInterface
-     */
-    private function isJson($stream): bool
+    private function isJson(StreamInterface $stream): bool
     {
+        if (!function_exists('json_decode')) {
+            return false;
+        }
         $stream->rewind();
 
         json_decode($stream->getContents());
@@ -108,17 +108,18 @@ private function isJson($stream): bool
 
     /**
      * @param $stream StreamInterface
-     *
-     * @return \SimpleXMLElement|false
      */
-    private function isXml($stream)
+    private function isXml(StreamInterface $stream): bool
     {
+        if (!function_exists('simplexml_load_string')) {
+            return false;
+        }
         $stream->rewind();
 
         $previousValue = libxml_use_internal_errors(true);
         $isXml = simplexml_load_string($stream->getContents());
         libxml_use_internal_errors($previousValue);
 
-        return $isXml;
+        return false !== $isXml;
     }
 }
diff --git a/src/Plugin/CookiePlugin.php b/src/Plugin/CookiePlugin.php
index 8f8d5b0..8399210 100644
--- a/src/Plugin/CookiePlugin.php
+++ b/src/Plugin/CookiePlugin.php
@@ -91,15 +91,13 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
     /**
      * Creates a cookie from a string.
      *
-     * @param $setCookie
-     *
      * @return Cookie|null
      *
      * @throws TransferException
      */
-    private function createCookie(RequestInterface $request, $setCookie)
+    private function createCookie(RequestInterface $request, string $setCookieHeader)
     {
-        $parts = array_map('trim', explode(';', $setCookie));
+        $parts = array_map('trim', explode(';', $setCookieHeader));
 
         if (empty($parts) || !strpos($parts[0], '=')) {
             return null;
@@ -169,11 +167,9 @@ private function createCookie(RequestInterface $request, $setCookie)
     /**
      * Separates key/value pair from cookie.
      *
-     * @param $part
-     *
-     * @return array
+     * @param string $part A single cookie value in format key=value
      */
-    private function createValueKey($part)
+    private function createValueKey(string $part): array
     {
         $parts = explode('=', $part, 2);
         $key = trim($parts[0]);
diff --git a/src/Plugin/DecoderPlugin.php b/src/Plugin/DecoderPlugin.php
index b1ede5d..271ad3c 100644
--- a/src/Plugin/DecoderPlugin.php
+++ b/src/Plugin/DecoderPlugin.php
@@ -68,12 +68,8 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
 
     /**
      * Decode a response body given its Transfer-Encoding or Content-Encoding value.
-     *
-     * @param ResponseInterface $response Response to decode
-     *
-     * @return ResponseInterface New response decoded
      */
-    private function decodeResponse(ResponseInterface $response)
+    private function decodeResponse(ResponseInterface $response): ResponseInterface
     {
         $response = $this->decodeOnEncodingHeader('Transfer-Encoding', $response);
 
@@ -86,13 +82,8 @@ private function decodeResponse(ResponseInterface $response)
 
     /**
      * Decode a response on a specific header (content encoding or transfer encoding mainly).
-     *
-     * @param string            $headerName Name of the header
-     * @param ResponseInterface $response   Response
-     *
-     * @return ResponseInterface A new instance of the response decoded
      */
-    private function decodeOnEncodingHeader($headerName, ResponseInterface $response)
+    private function decodeOnEncodingHeader(string $headerName, ResponseInterface $response): ResponseInterface
     {
         if ($response->hasHeader($headerName)) {
             $encodings = $response->getHeader($headerName);
@@ -123,11 +114,9 @@ private function decodeOnEncodingHeader($headerName, ResponseInterface $response
     /**
      * Decorate a stream given an encoding.
      *
-     * @param string $encoding
-     *
      * @return StreamInterface|false A new stream interface or false if encoding is not supported
      */
-    private function decorateStream($encoding, StreamInterface $stream)
+    private function decorateStream(string $encoding, StreamInterface $stream)
     {
         if ('chunked' === strtolower($encoding)) {
             return new Encoding\DechunkStream($stream);