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 Not

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');
}