public static function header($data) {
		if (empty($data['response'])) {
			return null;
		}
		if (isset($data['nonce'])) {
			$defaults =  [
				'realm' => 'app', 'method' => 'GET', 'uri' => '/',
				'username' => null, 'qop' => 'auth',
				'nonce' => null, 'opaque' => null,
				'cnonce' => md5(time()),  'nc' => static::$nc
			];
			$data += $defaults;
			$auth = "username=\"{$data['username']}\", response=\"{$data['response']}\", ";
			$auth .= "uri=\"{$data['uri']}\", realm=\"{$data['realm']}\", ";
			$auth .= "qop=\"{$data['qop']}\", nc={$data['nc']}, cnonce=\"{$data['cnonce']}\", ";
			$auth .= "nonce=\"{$data['nonce']}\", opaque=\"{$data['opaque']}\"";
			return "Digest " . $auth;
		}
		return "Basic " . $data['response'];
	}