php undefined index: username,PHP Undefined index: HTTP_USER_AGENT

The following code validates the user agent accessing the site however I am getting the error. What do I need to update to accommodate scenarios where there is no user agent being set?

ERROR PHP Notice: Undefined index: HTTP_USER_AGENT in Utils.php on line 7

CODE public static function detectBrowser() { $userAgent = strtolower($_SERVER['HTTP_USER_AGENT']); if (preg_match('/opera/', $userAgent)) { $name = 'opera'; } elseif (preg_match('/webkit/', $userAgent)) { $name = 'safari'; } elseif (preg_match('/msie/', $userAgent)) { $name = 'msie'; } elseif (preg_match('/mozilla/', $userAgent) && !preg_match('/compatible/', $userAgent)) { $name = 'mozilla'; } else { $name = 'unrecognized'; } if (preg_match('/.+(?:rv|it|ra|ie)[\/: ]([\d.]+)/', $userAgent, $matches)) { $version = $matches[1]; } else { $version = 'unknown'; } if (preg_match('/linux/', $userAgent)) { $platform = 'linux'; } elseif (preg_match('/macintosh|mac os x/', $userAgent)) { $platform = 'mac'; } elseif (preg_match('/windows|win32/', $userAgent)) { $platform = 'windows'; } else { $platform = 'unrecognized'; } return array( 'name' => $name, 'version' => $version, 'platform' => $platform, 'userAgent' => $userAgent ); }