Mobile Detect is a lightweight PHP class for detecting mobile devices (including tablets). It uses the User-Agent string combined with specific HTTP headers to detect the mobile environment.

You may consider this script as being part of the RESS (Responsive Web Design with Server-Side Component) movement. You can find out more on the topic by reading these articles: Improve Mobile Support With Server-Side-Enhanced Responsive Design and RESS: Responsive Design + Server Side Components.

You can also benefit from Mobile Detect by using any of the 3rd party plugins available for: Wordpress, Drupal, Joomla, Magento, etc.