1. Browsers Quirks – Despite the standardization in browsers, there will remain some quirks in various browsers that you will need to iron out on a regular basis.
2. Personalize Content – It may be required to deliver different type of content depending on the browser type (although it is usually not recommended); whether mobile or otherwise.
3. Illegal Access – Prevent bandwidth hogging bots and poorly programmed clients from downloading your content.

Below is a sample header for my site when using Firefox.

http://www.codediesel.com/
 
GET / HTTP/1.1
Host: www.codediesel.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.6)
Gecko/2009011913 Firefox/3.0.6 FirePHP/0.2.4
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive

PHP already has a builtin function – get_browser() that lets you determine the capabilities of the user’s browser, but it requires a browscap.ini to be installed, which sadly is not bundled with PHP, and also the function can slow down your php script. The other way to get a user agent is to use php server variables like below,

echo $_SERVER['HTTP_USER_AGENT'];

In this post we will use the Net_UserAgent_Detect Pear package to determine user agents.

Installation

Net_UserAgent_Detect being a Pear package we will use the Pear installer as below. I recommend to always use the Pear installer to download packages rather than downloading it manually as the Pear installer automatically downloads any dependent packages.

pear install Net_UserAgent_Detect-2.5.1

Usage

Usage is quite simple. For example the following code:

<?php
 
require_once 'Net/UserAgent/Detect.php';
 
echo Net_UserAgent_Detect::getUserAgent();
 
?>

will return the following string on Firefox 3.0.6: This can be quite useful if you need to log the users browser or OS type.

Mozilla/5.0 (Windows; U; Windows NT 5.1; 
en-US; rv:1.9.0.6) Gecko/2009011913 Firefox/3.0.6 FirePHP/0.2.4

getUserAgent() is only one of the many methods available in the class.

<?php
 
require_once 'Net/UserAgent/Detect.php';
 
echo('Browser String: '.Net_UserAgent_Detect::getBrowserString())."<br>";
echo('OS String: '.Net_UserAgent_Detect::getOSString())."<br>";
echo('Javascript: '.Net_UserAgent_Detect::getFeature('javascript'));
 
?>

Some other useful functions to directly test the type of browser are:
isIE()
isNavigator( )
isNetscape( )

So to detect if the user is running Firefox:

$browser = Net_UserAgent_Detect::isNetscape();