add phpstan and configuration

composer.json

@@ -32,7 +32,8 @@
     "require-dev": {
         "php": "^7.4",
         "phpunit/phpunit" : "^9.5",
-        "friendsofphp/php-cs-fixer": "^3.9"
+        "friendsofphp/php-cs-fixer": "^3.9",
+        "phpstan/phpstan": "^1.8"
     },
     "autoload": {
         "psr-4": {

phpstan.neon

@@ -0,0 +1,6 @@
+parameters:
+    paths:
+        - src
+        - Tests
+    level: 0
+    phpVersion: 70300