Add phpstan tool

.gitattributes

@@ -6,5 +6,6 @@
 /docs/ export-ignore
 /mkdocs.yml export-ignore
 /phpcs.xml export-ignore
+/phpstan.neon.dist export-ignore
 /phpunit.xml.dist export-ignore
 /test/ export-ignore

.gitignore

@@ -3,6 +3,7 @@
 /clover.xml
 /coveralls-upload.json
 /docs/html/
+/phpstan.neon
 /phpunit.xml
 /vendor/
 /zf-mkdoc-theme.tgz

composer.json

@@ -28,6 +28,7 @@
     },
     "require-dev": {
         "http-interop/http-middleware": "^0.4.1",
+        "phpstan/phpstan": "~0.11.1",
         "phpunit/phpunit": "^8.0",
         "webimpress/coding-standard": "dev-master",
         "zendframework/zend-coding-standard": "2.0.0-alpha.3",
@@ -77,6 +78,7 @@
         ],
         "cs-check": "phpcs",
         "cs-fix": "phpcbf",
+        "phpstan": "phpstan analyse",
         "test": "phpunit --colors=always",
         "test-coverage": "phpunit --colors=always --coverage-clover clover.xml"
     }