You could use either JavaScript or PHP to do this but I prefer PHP, as Nancy said JavaScript can be a bit shakey to say the least across different devices and it may not be enabled.

I have come across a few ways to this but my preferred choice is a project called MobileESP. This project offers a JavaScript or a PHP solution for detecting a massive array of different devices. Because of how complex this project is, I thought I'd explain a very basic way of using the scripts to redirect the client to the appropriate page dependant on what type of device the client is using. PHP is the best solution for this. This script was acquired from there site.

Go to http://blog.mobileesp.com/ and download the PHP version of MobileESP which is a file called mdetect.php.

Put this file in a folder called scripts.

Then for your index.php home page at the site root, add this script below,

<?php

include("/scripts/mdetect.php");

//Instantiate the object to do our testing with.

$uagent_obj = new uagent_info();

//Detect iPhone Tier and iPads...

if (($uagent_obj->DetectTierIphone() == $uagent_obj->true) ||

($uagent_obj->DetectIpad() == $uagent_obj->true))

  { header('Location: http://www.yoursite.com/i/'); } //Edit here

//Detect Rich CSS Tier...

else if ($uagent_obj->DetectTierRichCss() == $uagent_obj->true)

  { header('Location: http://www.yoursite.com/r/'); } //Edit here

//Detect All Other Mobile Devices...

else if ($uagent_obj->DetectTierOtherPhones() == $uagent_obj->true)

  { header('Location: http://www.yoursite.com/m/'); } //Edit here

//Else it's a regular PC browser -- send to regular desktop site

else

  { header('Location: http://www.yoursite.com/d/'); } //Edit here

?>

This script redirects iPhone Tier devices and iPads to www.yoursite.com/i/, Rich CSS devices such as Nokia N95 to www.yoursite.com/r/, Generic mobile devices to www.yoursite.com/m/ and Desktop browsers and all others to www.yoursite.com/d/.

You can also redirect to any file i.e. www.yoursite.com/d/index.html or www.yoursite.com/d/index.js.

Hopefully this should answer your question.