CodeIgniter is an open source rapid development web application framework, for use in building dynamic web sites with PHP.

Here are some useful tutorials for beginners.

http://www.youtube.com/watch?v=9hRNFgSLLAQ (a serises of 11 tutorials)

http://www.youtube.com/watch?v=KMg-pqwaGaE (a little bit advance series of 8 tutorials)