final public static function print_json(
		int $errno=0, $data=null, int $http_code=200, int $cache=0
	) {
		$json = json_encode(compact('errno', 'data'));
		self::start_header($http_code, $cache, [
			'Content-Length: ' . strlen($json),
			'Content-Type: application/json',
		]);
		static::halt($json);
	}