Install PHPStan package

Author: akalongman

composer.json

@@ -33,7 +33,8 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.7|^6.5|^7.4",
-        "squizlabs/php_codesniffer": "^3.3"
+        "squizlabs/php_codesniffer": "^3.3",
+        "phpstan/phpstan": "^0.10.5"
     },
     "autoload": {
         "psr-4": {