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

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.