public function addHeader($key, $value) {

		if(false === is_string($key)) {
			return trigger_error(sprintf('Argument 1 passed to %s() must be of the type string, "%s" given', __METHOD__, gettype($key)), E_USER_ERROR);
		}

		if(false === is_string($value)) {
			return trigger_error(sprintf('Argument 2 passed to %s() must be of the type string, "%s" given', __METHOD__, gettype($value)), E_USER_ERROR);
		}

		if(false === isset($this -> headers['custom'])) {
			$this -> headers['custom'] = [];
		}

		$this -> headers['custom'][$key] = $value;
	}