It is recommended that you use the official PHP API provided by YouTube at https://github.com/google/google-api-php-client
This library will let a user authenticate with youtube and perform actions such as uploading a video, commenting and rating videos, and getting video feeds.
Some basic documentation can be found at http://code.google.com/apis/youtube/articles/codeigniter_library.html For the most up to date documentation checkout my blog at http://jimdoescode.blogspot.com
Copy the files under your application directory. Then load the library like this:
$params['apikey'] = 'YOUTUBE API KEY';
$this->load->library('youtube', $params);
$this->youtube->getMostViewedVideoFeed(array('max-results'=>30));
This library is licensed under the MIT license.
You can also use this library with Sparks. Simply install using sparks then call.
$this->load->spark('youtube/1.0.0');
Then load the library as specified in the usage.