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

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.