 [ENHANCEMENT] Static analysers/linters for code consistency and unit testing

```
$ sudo apt install composer php-tokenizer
$ composer require --dev "squizlabs/php_codesniffer=*"
$ ./vendor/bin/phpcs

```

```
 $ composer require --dev phpstan/phpstan
 ./vendor/bin/phpstan analyse src
```

#963 for example was found using this tooling. Would be an good addition to CI testing.

