Fix issues reported by phpstan.

.travis.yml

@@ -24,6 +24,9 @@ matrix:
     - php: 7.0
       env: PHPCS=1 DEFAULT=0
 
+    - php: 7.0
+        env: PHPSTAN=1 DEFAULT=0
+
 before_script:
   - if [[ $TRAVIS_PHP_VERSION != 7.0 ]]; then phpenv config-rm xdebug.ini; fi
 
@@ -38,8 +41,11 @@ before_script:
 script:
   - if [[ $DEFAULT = 1 && $TRAVIS_PHP_VERSION != 7.0 ]]; then vendor/bin/phpunit; fi
   - if [[ $DEFAULT = 1 && $TRAVIS_PHP_VERSION = 7.0 ]]; then vendor/bin/phpunit --coverage-clover=clover.xml; fi
+
   - if [[ $PHPCS = 1 ]]; then vendor/bin/phpcs -n -p --extensions=php --standard=vendor/cakephp/cakephp-codesniffer/CakePHP ./src ./tests; fi
 
+  - if [[ $PHPSTAN = 1 ]]; then composer require --dev phpstan/phpstan:^0.8 && vendor/bin/phpstan analyse -l 5 src; fi
+
 after_success:
   - if [[ $DEFAULT = 1 && $TRAVIS_PHP_VERSION = 7.0 ]]; then bash <(curl -s https://codecov.io/bash); fi
 

src/Model/Behavior/SlugBehavior.php

@@ -79,9 +79,9 @@ class SlugBehavior extends Behavior
     ];
 
     /**
-     * Slugger instance
+     * Slugger instance or callable
      *
-     * @var \Muffin\Slug\SluggerInterface
+     * @var \Muffin\Slug\SluggerInterface|callable
      */
     protected $_slugger;
 
@@ -124,16 +124,16 @@ public function initialize(array $config)
     /**
      * Get/set slugger instance.
      *
-     * @param callable $slugger Sets slugger instance if passed.
+     * @param \Muffin\Slug\SluggerInterface|callable $slugger Sets slugger instance if passed.
      *   If no argument is passed return slugger intance based on behavior config.
-     * @return callable|void
+     * @return callable|\Muffin\Slug\SluggerInterface|null
      */
     public function slugger($slugger = null)
     {
         if ($slugger !== null) {
             $this->_slugger = $slugger;
 
-            return;
+            return null;
         }
 
         if ($this->_slugger !== null) {