Threads such as PHP Session Fixation / Hijacking and some people like Chris Shiflett recommend checking the user agent ( $_SESSION['HTTP_USER_AGENT'] ) to help check for session validity. Some resources even recommend something like this:
<?php
$string = $_SERVER['HTTP_USER_AGENT'];
$string .= 'SHIFLETT';
/* Add any other data that is consistent */
$fingerprint = md5($string);
?>
However, Chris Snyder says that "the universe of browser agents is miniscule in comparison to the universe of users, so it is impossible for each user to have an individual user agent. Furthermore, it isn't hard to spoof a user agent. And so there is little real point in checking this metric as a proof of session validity" (Chapter 7, pg 103).
It's very difficult to know what to do when one encounters conflicting advice, and when some of the advice may be out-of-date (such as the Shiflett/PHPSec example above, whose timestamp seems to be Friday, March 18, 2005). Newer advice such as Snyder's (date of publication: December 9, 2010) would seem to be better, but is this always so? (For example, in spite of spending a lot of time recommending the use of mysqli, Snyder completely ignores what Stack Overflow users seem to agree is the better choice -- PDO -- so I'm not totally sold on Snyder as the ultimate Trustworthy Expert).
So I guess my question has two parts, one specific (should I bother examining the user agent?) and one more general (whose advice should I trust when it comes to the latest thinking in PHP Security?), with my obvious bias being "trust the people on Stack Overflow!" -- or I wouldn't be asking in the first place, because crowd-sourcing the most current thinking is often the best idea.
Following useful discussion in the comments w/ @Radu, to clarify the HTTPS question --
Snyder seems to be saying two things: 1.) HTTPS makes other tools less necessary or unnecessary. 2.) In situations where one cannot use HTTPS, it is still not really useful to check the user agent (and this seems to be the point where he disagrees with some possibly older advice).
If the man in the middle can hijack the session ID, then he should have absolutely no problem in sending the same user agent, so I don't think this will get you anywhere. This is security by obscurity.
If you want real protection, use HTTPS.