public function filterEnv($variableName) {
        if(!is_string($variableName)) {
            throw new \Exception('Variable name expected as string');
        }
        
        if(!$this->filterHas(\INPUT_ENV, $variableName)) {
            return null;
        }
        
        // Sadly INPUT_ENV is broken and does not work
        // getenv() has to be used
        return $this->filter(getenv($variableName));
    }