You can gather a lot of information about a person’s computer by using $_SERVER[‘HTTP_USER_AGENT’]. This can tell us more about the user’s operating system, as well as their browser.
This can be useful to programmers if they are using special features that may not work for everyone, or if they want to get an idea of their target audience. Example :
<?php echo $_SERVER['HTTP_USER_AGENT'] ?>
Mozilla/5.0 (Windows NT 6.1; rv:22.0) Gecko/20100101 Firefox/22.0