Fix PHPStan complains

composer.json

@@ -10,7 +10,8 @@
         "email": "yago.riveiro@gmail.com"
     }],
     "require": {
-        "php": ">=7.0"
+        "php": ">=7.0",
+        "phpstan/phpstan": "^0.9.1"
     },
     "require-dev": {
         "phpunit/phpunit": "~6.2",
@@ -27,4 +28,3 @@
         }
     }
 }
-

src/Backoff.php

@@ -29,11 +29,11 @@ public function __construct($options = [])
     /**
      * Returns an array of Configuration.
      *
-     * cap:         Max duration allowed (in microseconds). If backoff duration
-     *              is greater than cap, cap is returned.
+     * cap:          Max duration allowed (in microseconds). If backoff duration
+     *               is greater than cap, cap is returned.
      * maxAttempts:  Number of attemps before thrown an Yriveiro\Backoff\BackoffException.
      *
-     * @return mixed
+     * @return array
      */
     public static function getDefaultOptions() : array
     {
@@ -46,9 +46,11 @@ public static function getDefaultOptions() : array
     /**
      * Allows overwrite default option values.
      *
-     * @param mixed
+     * @param array $options Configuration options.
+     *
+     * @return BackoffInterface
      */
-    public function setOptions($options)
+    public function setOptions($options) : BackoffInterface
     {
         if (!is_array($options)) {
             $options = [$options];

tests/BackoffTest.php

@@ -10,6 +10,8 @@
 
 class BackoffTest extends TestCase
 {
+    protected $backoff;
+
     public function setUp()
     {
         $this->backoff = new Backoff();