diff --git a/.travis.yml b/.travis.yml
index 32d71d3..01cfd22 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -20,6 +20,7 @@ script:
   - if [[ "$TRAVIS_PHP_VERSION" == '7.0' ]]; then mkdir -p build/logs && phpunit --coverage-clover build/logs/clover.xml ; fi
   - if [[ "$TRAVIS_PHP_VERSION" != '7.0' ]]; then vendor/bin/phpunit ; fi
   - if [[ "$TRAVIS_PHP_VERSION" == '7.0' ]]; then vendor/bin/phpcs ; fi
+  - vendor/bin/phpstan analyse -l 3 src/
 
 after_script:
   - if [[ "$TRAVIS_PHP_VERSION" == '7.0' ]]; then php vendor/bin/coveralls -v ; fi
diff --git a/composer.json b/composer.json
index b1c83e3..bd88413 100644
--- a/composer.json
+++ b/composer.json
@@ -29,7 +29,8 @@
   "require-dev": {
     "squizlabs/php_codesniffer": "^2.5",
     "phpunit/phpunit": "^6.0|^7.0",
-    "php-http/psr7-integration-tests": "dev-master"
+    "php-http/psr7-integration-tests": "dev-master",
+    "phpstan/phpstan": "^0.9"
   },
   "provide": {
     "psr/http-message-implementation": "1.0"
diff --git a/src/Cookies.php b/src/Cookies.php
index a1c01df..2dfbad7 100644
--- a/src/Cookies.php
+++ b/src/Cookies.php
@@ -156,7 +156,7 @@ protected function toHeader($name, array $properties)
      * Parse HTTP request `Cookie:` header and extract
      * into a PHP associative array.
      *
-     * @param  string $header The raw HTTP request `Cookie:` header
+     * @param  string|string[] $header The raw HTTP request `Cookie:` header
      *
      * @return array Associative array of cookie names and values
      *
diff --git a/src/Request.php b/src/Request.php
index f6ff5f7..9144fe3 100644
--- a/src/Request.php
+++ b/src/Request.php
@@ -74,14 +74,14 @@ class Request extends Message implements ServerRequestInterface
     /**
      * The request attributes (route segment names and values)
      *
-     * @var \Slim\Collection
+     * @var Collection
      */
     protected $attributes;
 
     /**
      * The request body parsed (if possible) into a PHP array or object
      *
-     * @var null|array|object
+     * @var null|array|object|false
      */
     protected $bodyParsed = false;
 
@@ -263,13 +263,13 @@ public function withMethod($method)
      * Validate the HTTP method
      *
      * @param  null|string $method
-     * @return null|string
+     * @return string
      * @throws \InvalidArgumentException on invalid HTTP method.
      */
     protected function filterMethod($method)
     {
         if ($method === null) {
-            return $method;
+            return '';
         }
 
         if (!is_string($method)) {
@@ -1051,7 +1051,7 @@ public function registerMediaTypeParser($mediaType, callable $callable)
         if ($callable instanceof Closure) {
             $callable = $callable->bindTo($this);
         }
-        $this->bodyParsers[(string)$mediaType] = $callable;
+        $this->bodyParsers[$mediaType] = $callable;
     }
 
     /*******************************************************************************
diff --git a/src/Stream.php b/src/Stream.php
index 5be9c96..f49d176 100644
--- a/src/Stream.php
+++ b/src/Stream.php
@@ -40,35 +40,35 @@ class Stream implements StreamInterface
     /**
      * The underlying stream resource
      *
-     * @var resource
+     * @var resource|null
      */
     protected $stream;
 
     /**
      * Stream metadata
      *
-     * @var array
+     * @var array|null
      */
     protected $meta;
 
     /**
      * Is this stream readable?
      *
-     * @var bool
+     * @var bool|null
      */
     protected $readable;
 
     /**
      * Is this stream writable?
      *
-     * @var bool
+     * @var bool|null
      */
     protected $writable;
 
     /**
      * Is this stream seekable?
      *
-     * @var bool
+     * @var bool|null
      */
     protected $seekable;
 
@@ -82,7 +82,7 @@ class Stream implements StreamInterface
     /**
      * Is this stream a pipe?
      *
-     * @var bool
+     * @var bool|null
      */
     protected $isPipe;
 
diff --git a/src/Uri.php b/src/Uri.php
index 6712a5a..40ee88c 100644
--- a/src/Uri.php
+++ b/src/Uri.php
@@ -275,7 +275,7 @@ protected function filterScheme($scheme)
             throw new InvalidArgumentException('Uri scheme must be a string');
         }
 
-        $scheme = str_replace('://', '', strtolower((string)$scheme));
+        $scheme = str_replace('://', '', strtolower($scheme));
         if (!isset($valid[$scheme])) {
             throw new InvalidArgumentException('Uri scheme must be one of: "", "https", "http"');
         }
@@ -656,7 +656,7 @@ public function withQuery($query)
         if (!is_string($query) && !method_exists($query, '__toString')) {
             throw new InvalidArgumentException('Uri query must be a string');
         }
-        $query = ltrim((string)$query, '?');
+        $query = ltrim($query, '?');
         $clone = clone $this;
         $clone->query = $this->filterQuery($query);
 
@@ -724,7 +724,7 @@ public function withFragment($fragment)
         if (!is_string($fragment) && !method_exists($fragment, '__toString')) {
             throw new InvalidArgumentException('Uri fragment must be a string');
         }
-        $fragment = ltrim((string)$fragment, '#');
+        $fragment = ltrim($fragment, '#');
         $clone = clone $this;
         $clone->fragment = $this->filterQuery($fragment);