With more requirement of responsive websites it a basic requirement to detect device and browser of user accessing website. Here is an opensource class in PHP to detect device as well OS/browser and other features including tablets.

Plugins of this class are also available for WordPress, Joomla, Magento.

Demo:
http://demo.mobiledetect.net

Download:
http://mobiledetect.net

If you don’t wish to go much into code and need a quick solution, an alternate can also be:

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Thanks to an unknown author for above quick fix. : )