How websites can detect which mobile phone is viewing the website?

There is a free PHP script available on the net that detects which phone is viewing the website.

Reference URL: http://detectmobilebrowsers.mobi/

- What are the Guidelines to create a website for mobile phone?

A very good article is available on the net. Please check the below URL for details.

Reference URL: http://www.webcredible.co.uk/user-friendly-resources/web-usability/mobile-guidelines.shtml