Is it possible for $_SERVER['HTTP_USER_AGENT'] to not be set?

Question

I've just been looking through a website's error_log and one of the error's that has been logged a few times is:

[21-Jun-2011 12:24:03] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/ukevents/public_html/lib/toro.php on line 130

The line this pertains to in toro.php is:

private function ipad_request() {
    return strstr($_SERVER['HTTP_USER_AGENT'], 'iPad');
}

Is it possible for $_SERVER['HTTP_USER_AGENT'] to not be set by a HTTP request?

Answer 1

Yes, it's possible, this a HTTP header sent (or not sent) by client, and you should not rely on it. From php manual:

Contents of the User-Agent: header from the current request, if there is one

So the correct code would be:

private function ipad_request() {
    return isset($_SERVER['HTTP_USER_AGENT']) && strstr($_SERVER['HTTP_USER_AGENT'], 'iPad');
}

Answer 2

Yes. Any browser or user-agent can choose not to send the User-Agent header. If they do not send that header, $_SERVER['HTTP_USER_AGENT'] won't be set.

Use isset() to ensure that $_SERVER['HTTP_USER_AGENT'] is set.

private function ipad_request() {
  if(!isset($_SERVER['HTTP_USER_AGENT'])) return false;

  return strstr($_SERVER['HTTP_USER_AGENT'], 'iPad') !== false;
}

Answer 3

Yes, it's possible, but it never happens for a regular request.

All browsers do send a browser string in the request, so any request that arrives without one comes from some other program. Even all well-behaving bots send a browser string, so you don't have to be concerned about not showing up in search engines either.

Answer 4

PHP docs says:

'HTTP_USER_AGENT' Contents of the User-Agent: header from the current request, if there is one.

(relevant part italicised) so it would appear it might not always be set.

Answer 5

An example where HTTP_USER_AGENT is undefined is if the request coming from GoDaddy's 404 page handler for your site where you have set the handler to be one of your pages.